Be a bit more exacting with error messages when parsing numbers.

In particular, the lexer has no idea about the range of numbers,
so we shouldn't claim that there is a bug in the program if we
can't handle an integer that can't fit into 64 bits. Similarly
with floats.
Whoopsies. Forgot to replace escape sequences with actual characters.
Merge branch 'master' of github.com:BurntSushi/toml