Fix split_at (return Result ver.)

[kyoheiu]

Fixed #453

See also another PR #456 .

This PR fixes split_at, which can panic when splitting occurs inside the middle of a wide character, to return Result to avoid panic.
Also includes some of 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-result

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

which means nothing is changed by this PR except split_at and its test.

[Enselic]

To you make your PRs easier to review, can you undo the formatting changes please?

(IMHO we should enforce cargo fmt in CI, which is a widely used practice across the Rust ecosystem, and works great in my experience. I would volunteer to set it up if there is interest.)

[kyoheiu]

Sorry for the inconvenience: Reverted the formatting in 2 PRs I created.

[Enselic]

Sorry for keeping you waiting.

Before we can make progress here, we need to make CI green. I created a PR for that: #462

The test failures you see are because you did not clone submodules. Note in .github/workflows/CI.yml how submodules: true for actions/checkout@v2 in build-and-test.

Try running git submodule update --init before you run cargo test.