Use standard library for float from_str_radix with radix 10

[tspiteri]

Fixes #198.

While at it, also make parsing of the "-NaN" string match the standard library.

[tspiteri]

#198 (comment) says "it would be nicer to improve this for all bases", but that is a lot of work with basically zero usefulness, which is the reason from_str_radix was removed from the standard library floating-point numbers.

[cuviper]

Thanks!

@bors r+

[cuviper]

bors r+

[bors]

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