Use error hints for convert and (::Type)(args...) MethodErrors

[MasonProtter]

Currently, we have some special-cased logic in the printing of MethodError that combines the method candidates of (::Type)(args...) with the method candidates for convert(::Type, args...). This is because these things are frequently confused with eachother, but IMO it's a bit misleading because those are not actually candidates!

i.e.

julia> struct Foo; x::Int; end

julia> convert(Foo, 1)
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type Foo
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  Foo(::Int64)
   @ Main REPL[3]:2
  Foo(::Any)
   @ Main REPL[3]:2
  convert(::Type{T}, ::T) where T
   @ Base Base_compiler.jl:133

Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

Foo(::Int) and Foo(::Any) weren't actually dispatch candidates here and it's kinda weird to list them.

---

The current implementation preceeds us having error hints, but I think this is a good use-case for those hints. This PR uses hints to turn the error messages into

julia> convert(Foo, 1)
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type Foo
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  convert(::Type{T}, ::T) where T
   @ Base Base_compiler.jl:136

Hint: Did you mean to call Foo(::Int64) ?

Stacktrace:
 [1] top-level scope
   @ REPL[1]:1

and likewise, when the user calls a type on arguments, if there's an applicable convert method, we also suggest it:

julia> Base.convert(::Type{String}, f::Foo) = string(f);

julia> String(Foo(1))
ERROR: MethodError: no method matching String(::Foo)
The type `String` exists, but no method is defined for this combination of argument types when trying to construct it.

Closest candidates are:
  String(::LazyString)
   @ Base strings/lazy.jl:80
  String(::Vector{UInt8})
   @ Base strings/string.jl:66
  String(::SubString{String})
   @ Base strings/substring.jl:75
  ...

Hint: Did you mean to call convert(String, ::Foo) ?

Stacktrace:
 [1] top-level scope
   @ REPL[3]:1