Use `lexical-core` for performance / accuracy

[davidhewitt]

In #51 we looked at using lexical-core in one specific case and decided instead to go with #52 for simplicity for now.

However in principle lexical-core should be more accurate for float parsing, which seems worth moving forward with sometime.

Also we should try some benchmark using lexical-core for integers instead of our hand-rolled implementations in numbers.rs; if it's similar (or hopefully better) then it's worth switching for integers too.