Converted the floating point types to the new string conversion functions.

Also fixed all conflicting calls of the old functions in the rest of the codebase
EXCEPT that now one test fails: libcore/repr.rs fails on line 584 because the
to_str() method now calculates all possible digits, instead of maximum 4.

The set of string conversion functions for each of them now consists of those items:
- to_str(), converts to number in base 10
- to_str_hex(), converts to number in base 16
- to_str_radix(), converts to number in given radix
- to_str_exact(), converts to number in base 10 with a exact number of trailing digits
- to_str_digits(), converts to number in base 10 with a maximum number of trailing digits
- implementations for to_str::ToStr and num::ToStrRadix
- from_str(), parses a string as number in base 10 including decimal exponent and special values
- from_str_hex(), parses a string as a number in base 16 including binary exponent and special values
- from_str_radix(), parses a string as a number in a given base excluding any exponent and special values
- implementations for from_str::FromStr and num::FromStrRadix