Add "partial" searching methods that don't match a substring at end of input that could be extended to a bigger match

[jwodder]

• E.g., searching for {CR, CRLF} will not match a terminating '\r', as it could become a CRLF if a '\n' were appended.

• Similarly, when matching paragraph separators, multiple newlines at end of input are not matched, as adding more characters could extend the separator.

• This may be useful for Add a decoder-like type for splitting incrementally-received text #10.

• The methods should probably return Option<PartialMatch>, where PartialMatch has FullMatch {start: usize, end: usize} and PotentialStartOfLongerMatch(usize) variants (All names not final)