Reduce allocations in Find and ReplaceAll

[pgavlin]

• Ensure that findIter and overlappingIter do not escape to the heap in Next
• Have FindAll use a findIter directly rather than using an Iter
• Do not allocate a slice of replacements in ReplaceAll
• Right-size the output buffer in ReplaceAll

[pgavlin]

benchstat output:

aho-corasick ❯ benchstat base.txt diff.txt 
goos: darwin
goarch: arm64
pkg: github.com/petar-dambovaliev/aho-corasick
cpu: Apple M4 Max
                                            │  base.txt   │              diff.txt               │
                                            │   sec/op    │   sec/op     vs base                │
Stdlib_StringsReplaceAll/No_matches-16        57.46µ ± 1%   57.26µ ± 1%        ~ (p=0.247 n=10)
Stdlib_StringsReplaceAll/Matches-16           52.37µ ± 1%   51.17µ ± 0%   -2.28% (p=0.000 n=10)
Stdlib_AhoCorasickReplaceAll/No_matches-16    45.14µ ± 1%   44.06µ ± 1%   -2.40% (p=0.000 n=10)
Stdlib_AhoCorasickReplaceAll/Matches-16       55.28µ ± 1%   48.69µ ± 1%  -11.92% (p=0.000 n=10)
AhoCorasick_ReplaceAllDFA-16                  53.32µ ± 2%   45.31µ ± 1%  -15.01% (p=0.000 n=10)
AhoCorasick_ReplaceAllNFA-16                  59.46µ ± 2%   50.90µ ± 2%  -14.41% (p=0.000 n=10)
AhoCorasick_LeftmostInsensitiveWholeWord-16   49.29µ ± 1%   47.03µ ± 1%   -4.59% (p=0.000 n=10)
geomean                                       52.99µ        49.04µ        -7.46%
                                            │   base.txt   │                diff.txt                │
                                            │     B/op     │     B/op      vs base                  │
Stdlib_StringsReplaceAll/No_matches-16        112.0Ki ± 0%   112.0Ki ± 0%        ~ (p=1.000 n=10) ¹
Stdlib_StringsReplaceAll/Matches-16           112.0Ki ± 0%   112.0Ki ± 0%        ~ (p=1.000 n=10) ¹
Stdlib_AhoCorasickReplaceAll/No_matches-16    56.11Ki ± 0%   56.05Ki ± 0%   -0.11% (p=0.000 n=10)
Stdlib_AhoCorasickReplaceAll/Matches-16       180.3Ki ± 0%   112.6Ki ± 0%  -37.53% (p=0.000 n=10)
AhoCorasick_ReplaceAllDFA-16                  181.7Ki ± 0%   113.6Ki ± 0%  -37.49% (p=0.000 n=10)
AhoCorasick_ReplaceAllNFA-16                  181.5Ki ± 0%   113.4Ki ± 0%  -37.51% (p=0.000 n=10)
AhoCorasick_LeftmostInsensitiveWholeWord-16   56.88Ki ± 0%   56.75Ki ± 0%   -0.22% (p=0.000 n=10)
geomean                                       113.2Ki        92.49Ki       -18.29%
¹ all samples are equal
                                            │  base.txt  │               diff.txt               │
                                            │ allocs/op  │ allocs/op   vs base                  │
Stdlib_StringsReplaceAll/No_matches-16        3.000 ± 0%   3.000 ± 0%        ~ (p=1.000 n=10) ¹
Stdlib_StringsReplaceAll/Matches-16           3.000 ± 0%   3.000 ± 0%        ~ (p=1.000 n=10) ¹
Stdlib_AhoCorasickReplaceAll/No_matches-16    3.000 ± 0%   2.000 ± 0%  -33.33% (p=0.000 n=10)
Stdlib_AhoCorasickReplaceAll/Matches-16       23.00 ± 0%   13.00 ± 0%  -43.48% (p=0.000 n=10)
AhoCorasick_ReplaceAllDFA-16                  81.00 ± 0%   56.00 ± 0%  -30.86% (p=0.000 n=10)
AhoCorasick_ReplaceAllNFA-16                  64.00 ± 0%   39.00 ± 0%  -39.06% (p=0.000 n=10)
AhoCorasick_LeftmostInsensitiveWholeWord-16   21.00 ± 0%   19.00 ± 0%   -9.52% (p=0.000 n=10)
geomean                                       13.14        9.958       -24.21%
¹ all samples are equal