Fix split_at (decrease index)

[kyoheiu]

Fixed #453

See also another PR #455 .

This PR fixes split_at, which can panic when splitting occurs inside the middle of a wide character, to decrese index to avoid panic (In this version, split_at returns the same type as before).
Also includes rust-analyzer's formatting.

Test results:
commit 8678b7c

failures:
    highlighting::highlighter::tests::can_parse
    highlighting::highlighter::tests::can_parse_with_highlight_state_from_cache
    highlighting::highlighter::tests::test_ranges
    highlighting::theme_set::tests::can_parse_common_themes
    html::tests::strings
    html::tests::tricky_test_syntax
    parsing::parser::tests::can_compare_parse_states
    parsing::parser::tests::can_parse_backrefs
    parsing::parser::tests::can_parse_includes
    parsing::parser::tests::can_parse_issue25
    parsing::parser::tests::can_parse_preprocessor_rules
    parsing::parser::tests::can_parse_simple
    parsing::parser::tests::can_parse_yaml
    parsing::syntax_set::tests::can_load

test result: FAILED. 86 passed; 14 failed; 0 ignored; 0 measured; 0 filtered out; finished in 3.11s

fix-split_at-change-index

failures:
    highlighting::highlighter::tests::can_parse
    highlighting::highlighter::tests::can_parse_with_highlight_state_from_cache
    highlighting::highlighter::tests::test_ranges
    highlighting::theme_set::tests::can_parse_common_themes
    html::tests::strings
    html::tests::tricky_test_syntax
    parsing::parser::tests::can_compare_parse_states
    parsing::parser::tests::can_parse_backrefs
    parsing::parser::tests::can_parse_includes
    parsing::parser::tests::can_parse_issue25
    parsing::parser::tests::can_parse_preprocessor_rules
    parsing::parser::tests::can_parse_simple
    parsing::parser::tests::can_parse_yaml
    parsing::syntax_set::tests::can_load

test result: FAILED. 86 passed; 14 failed; 0 ignored; 0 measured; 0 filtered out; finished in 3.00s

Nothing changed.

[trishume]

I'll use this version so it doesn't require a major version bump. Thanks and sorry it took so long for me to get around to this!