Use string builder to reduce allocations

[olivier403]

Hello, this pr uses strings.Builder instead of append to reduce the number of memory allocations.

Unidecode on string "üñíCöDÈ" repeated n times, with append:

cpu: AMD Ryzen 5 5600X 6-Core Processor             
BenchmarkUnidecode/go-unidecode_1-12             3358656               328.9 ns/op           256 B/op          7 allocs/op
BenchmarkUnidecode/go-unidecode_10-12             491061              2354 ns/op            4240 B/op         29 allocs/op
BenchmarkUnidecode/go-unidecode_100-12             64015             18734 ns/op           31440 B/op        212 allocs/op
BenchmarkUnidecode/go-unidecode_1000-12             4491            242756 ns/op          521266 B/op       2019 allocs/op
BenchmarkUnidecode/go-unidecode_10000-12             337           3411895 ns/op         5713300 B/op      20028 allocs/op

with strings.Builder:

cpu: AMD Ryzen 5 5600X 6-Core Processor             
BenchmarkUnidecode/go-unidecode_1-12            17364760                70.24 ns/op            8 B/op          1 allocs/op
BenchmarkUnidecode/go-unidecode_10-12            1610168               757.2 ns/op           248 B/op          5 allocs/op
BenchmarkUnidecode/go-unidecode_100-12            170352              6377 ns/op            1912 B/op          8 allocs/op
BenchmarkUnidecode/go-unidecode_1000-12            17743             68894 ns/op           34296 B/op         15 allocs/op
BenchmarkUnidecode/go-unidecode_10000-12            1668            760811 ns/op          285432 B/op         22 allocs/op

[mozillazg]

LGTM, Thanks for your contribution!