perf: Speed up identifier splitting

[epage]

Before

test process_code         ... bench:      25,627 ns/iter (+/- 2,062)
test process_corpus       ... bench:  20,192,253 ns/iter (+/- 603,029)
test process_empty        ... bench:       7,418 ns/iter (+/- 707)
test process_no_tokens    ... bench:       8,788 ns/iter (+/- 1,065)
test process_sherlock     ... bench:      30,420 ns/iter (+/- 2,699)
test process_single_token ... bench:       9,426 ns/iter (+/- 811)
test symbol_split_lowercase_long  ... bench:       2,763 ns/iter (+/- 246)
test symbol_split_lowercase_short ... bench:         110 ns/iter (+/- 12)
test symbol_split_mixed_long      ... bench:       7,373 ns/iter (+/- 1,111)
test symbol_split_mixed_short     ... bench:         357 ns/iter (+/- 86)

After

test process_code         ... bench:      20,973 ns/iter (+/- 1,717)
test process_corpus       ... bench:  15,826,059 ns/iter (+/- 1,016,628)
test process_empty        ... bench:       7,364 ns/iter (+/- 616)
test process_no_tokens    ... bench:       8,858 ns/iter (+/- 632)
test process_sherlock     ... bench:      24,707 ns/iter (+/- 3,482)
test process_single_token ... bench:       9,339 ns/iter (+/- 706)
test symbol_split_lowercase_long  ... bench:       2,727 ns/iter (+/- 151)
test symbol_split_lowercase_short ... bench:          46 ns/iter (+/- 2)
test symbol_split_mixed_long      ... bench:       5,753 ns/iter (+/- 441)
test symbol_split_mixed_short     ... bench:          76 ns/iter (+/- 3)

Fixes #33