Use StringMemory instead of StringVector where possible (#53962)

On `1.11.0-alpha2`
Old:
```julia
@benchmark Base.dec($0x1, $0, $false)
BenchmarkTools.Trial: 10000 samples with 994 evaluations.
 Range (min … max):  33.702 ns …   4.242 μs  ┊ GC (min … max):  0.00% … 97.61%
 Time  (median):     37.626 ns               ┊ GC (median):     0.00%
 Time  (mean ± σ):   45.787 ns ± 147.794 ns  ┊ GC (mean ± σ):  14.53% ±  4.47%

    ▄▅▆▇█▇▇▅▃▃▂▂▂▁    ▁▂▁▁▁             ▁▁   ▁                 ▂
  ▄▇███████████████▇▇██████▇█▆▆▄▄▃▄▅▄▆▇████████▆▅▅▇▆▅▆▄▄▅▄▄▄▁▅ █
  33.7 ns       Histogram: log(frequency) by time      67.5 ns <

 Memory estimate: 88 bytes, allocs estimate: 3.
```
New:
```julia
BenchmarkTools.Trial: 10000 samples with 995 evaluations.
 Range (min … max):  27.538 ns …   3.397 μs  ┊ GC (min … max):  0.00% … 97.86%
 Time  (median):     30.151 ns               ┊ GC (median):     0.00%
 Time  (mean ± σ):   34.547 ns ± 105.101 ns  ┊ GC (mean ± σ):  10.37% ±  3.39%

       ▁ █▆▃  ▁
  ▂▂▃▃▅█████▆████▆▄▄▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▁▂▂▂▂▂▁▂▂▂▂▂▂▂▂▂▂▂▂▂ ▃
  27.5 ns         Histogram: frequency by time         43.8 ns <

 Memory estimate: 56 bytes, allocs estimate: 2.
```

Fixes #53950, actually now even faster than `1.10.2`.

It doesn't look like the length is ever changed and we don't return
these `StringMemory`s so this change should be fine.

(cherry picked from commit 0e59c9e)

Author: Zentrik

base/gmp.jl

@@ -754,7 +754,7 @@ function string(n::BigInt; base::Integer = 10, pad::Integer = 1)
     iszero(n) && pad < 1 && return ""
     nd1 = ndigits(n, base=base)
     nd  = max(nd1, pad)
-    sv  = Base.StringVector(nd + isneg(n))
+    sv  = Base.StringMemory(nd + isneg(n))
     GC.@preserve sv MPZ.get_str!(pointer(sv) + nd - nd1, base, n)
     @inbounds for i = (1:nd-nd1) .+ isneg(n)
         sv[i] = '0' % UInt8

base/intfuncs.jl

@@ -742,7 +742,7 @@ ndigits(x::Integer; base::Integer=10, pad::Integer=1) = max(pad, ndigits0z(x, ba
 function bin(x::Unsigned, pad::Int, neg::Bool)
     m = top_set_bit(x)
     n = neg + max(pad, m)
-    a = StringVector(n)
+    a = StringMemory(n)
     # for i in 0x0:UInt(n-1) # automatic vectorization produces redundant codes
     #     @inbounds a[n - i] = 0x30 + (((x >> i) % UInt8)::UInt8 & 0x1)
     # end
@@ -769,7 +769,7 @@ end
 function oct(x::Unsigned, pad::Int, neg::Bool)
     m = div(top_set_bit(x) + 2, 3)
     n = neg + max(pad, m)
-    a = StringVector(n)
+    a = StringMemory(n)
     i = n
     while i > neg
         @inbounds a[i] = 0x30 + ((x % UInt8)::UInt8 & 0x7)
@@ -844,7 +844,7 @@ end
 
 function dec(x::Unsigned, pad::Int, neg::Bool)
     n = neg + ndigits(x, pad=pad)
-    a = StringVector(n)
+    a = StringMemory(n)
     append_c_digits_fast(n, x, a, 1)
     neg && (@inbounds a[1] = 0x2d) # UInt8('-')
     String(a)
@@ -853,7 +853,7 @@ end
 function hex(x::Unsigned, pad::Int, neg::Bool)
     m = 2 * sizeof(x) - (leading_zeros(x) >> 2)
     n = neg + max(pad, m)
-    a = StringVector(n)
+    a = StringMemory(n)
     i = n
     while i >= 2
         b = (x % UInt8)::UInt8
@@ -880,7 +880,7 @@ function _base(base::Integer, x::Integer, pad::Int, neg::Bool)
     b = (base % Int)::Int
     digits = abs(b) <= 36 ? base36digits : base62digits
     n = neg + ndigits(x, base=b, pad=pad)
-    a = StringVector(n)
+    a = StringMemory(n)
     i = n
     @inbounds while i > neg
         if b > 0
@@ -956,7 +956,7 @@ julia> bitstring(2.2)
 function bitstring(x::T) where {T}
     isprimitivetype(T) || throw(ArgumentError("$T not a primitive type"))
     sz = sizeof(T) * 8
-    str = StringVector(sz)
+    str = StringMemory(sz)
     i = sz
     @inbounds while i >= 4
         b = UInt32(sizeof(T) == 1 ? bitcast(UInt8, x) : trunc_int(UInt8, x))

base/strings/util.jl

@@ -1061,7 +1061,7 @@ function bytes2hex end
 
 function bytes2hex(itr)
     eltype(itr) === UInt8 || throw(ArgumentError("eltype of iterator not UInt8"))
-    b = Base.StringVector(2*length(itr))
+    b = Base.StringMemory(2*length(itr))
     @inbounds for (i, x) in enumerate(itr)
         b[2i - 1] = hex_chars[1 + x >> 4]
         b[2i    ] = hex_chars[1 + x & 0xf]

base/uuid.jl

@@ -90,7 +90,7 @@ let groupings = [36:-1:25; 23:-1:20; 18:-1:15; 13:-1:10; 8:-1:1]
     global string
     function string(u::UUID)
         u = u.value
-        a = Base.StringVector(36)
+        a = Base.StringMemory(36)
         for i in groupings
             @inbounds a[i] = hex_chars[1 + u & 0xf]
             u >>= 4

stdlib/FileWatching/src/pidfile.jl

@@ -282,7 +282,7 @@ function open_exclusive(path::String;
 end
 
 function _rand_filename(len::Int=4) # modified from Base.Libc
-    slug = Base.StringVector(len)
+    slug = Base.StringMemory(len)
     chars = b"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
     for i = 1:len
         slug[i] = chars[(Libc.rand() % length(chars)) + 1]