add case mapping fast paths for Latin-1

[ywxt]

Result:

Benchmark	Before (ns/iter)	After (ns/iter)	Improvement
bench_ascii_mix_to_lowercase	122,779.62	62,625.20	48.99%
bench_ascii_mix_to_uppercase	140,644.43	62,216.00	55.76%
bench_non_ascii_char_to_lowercase	178,361.80	62,815.57	64.78%
bench_non_ascii_char_to_uppercase	215,988.75	62,073.67	71.26%
bench_non_latin1_char_to_lowercase	296,788.33	293,113.17	-
bench_non_latin1_char_to_uppercase	348,730.00	347,126.50	-
bench_ascii_char_to_lowercase	65,546.00	62,120.53	-
bench_ascii_char_to_uppercase	65,532.87	62,126.94	-

r? @Amanieu

[rustbot]

library/core/src/unicode/unicode_data.rs is generated by the src/tools/unicode-table-generator tool.

If you want to modify unicode_data.rs, please modify the tool then regenerate the library source file via ./x run src/tools/unicode-table-generator instead of editing unicode_data.rs manually.