Improve ParseIndexError::InvalidCharacter error

[asmello]

In #80 I introduced a new error variant to capture cases where the array index starts with + (which is valid for usize representations in Rust, but not valid for array indices per RFC 6901).

As I noted here, keeping a collection of invalid characters in the error variant is overkill, since the accepted domain of usize::parse is very unlikely to evolve, as that'd be a breaking change in std. According to the docs + is the only accepted non-digit character, so we don't need to expect any others.

I'm not specialising the variant name for + though to leave some room for later adopting a different parser that has different domain (unlikely as that is) or in the off chance std does change this API. The error message changing isn't a hard break, if we ever need to update it in the future, so I'm keeping it more specific to make it more useful.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.13559% with 7 lines in your changes missing coverage. Please review.

Project coverage is 99.0%. Comparing base (c694765) to head (52f435a).

Files with missing lines	Patch %	Lines
src/index.rs	84.7%	7 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/assign.rs	99.6% <100.0%> (+<0.1%)	⬆️
src/index.rs	96.5% <84.7%> (-3.5%)	⬇️

... and 1 file with indirect coverage changes

[asmello]

I wonder if we should also make the validation more specific (to reduce the chance of the error message lying to the user), at the risk of accepting an invalid input we didn't expect to pass the usize::parse parsing. Probably overthinking it.