Add a color parse function consuming a string prefix

[tomcur]

The unparsed remainder of the string is indicated in the return value.

Resolves #60.

The existing function parse_color now returns ParseError::ExpectedEndOfString if there are trailing characters. Some choices could be made differently:

• For #f00fa the error ParseError::WrongNumberOfHexDigits is returned. It could instead parse as #f00f with trailing a. The current implementation is consistent with parsing ident to the longest string and then checking for matches.
• This drops ParseError::InvalidHexDigit, as it is ambiguous whether the character was intended as a hex digit or is the start of the trailing string. #fffffftr is parsed as #ffffff with trailing tr. This causes a slightly worse error for the full-string-consuming parse_color, namely ParseError::ExpectedEndOfString. We could have parse_color still return ParseError::InvalidHexDigit for that case, but then the two functions have slightly different parsing behavior, which is not ideal (the prefix parse might succeed, whereas the full parse fails with an error different than ExpectedEndOfString).

[tomcur]

We should link to the relevant spec sections, especially regarding the handling of trailing items after #rrggbbaa.

I'm not aware of an authoritative source on this, it's possible one exists, or perhaps there's prior art we can follow.

[DJMcNab]

Hmm. I presumed that the css specs would specify this. I can't imagine that e.g. border: #ffff00ff10px solid would be valid for a 10 pixel border. Although admittedly, I've not tried it.