Use parse_float-facilities from num-parse when available

[phorward]

Tokay uses the num-parse-crate to parse integers.

But there is also a pull request available which should add support to parse floating point numbers effectively using the same approach as parse_int.

Unfortunately, the pull request is stuck with a bug regarding precision, which is not equal to Rust's own compiler's implementation, but it should be equal as far as possible.

It would be nice to integrate num-parses parse_float into the Float-builtin token, which can directly read from a Reader.