lexical_cast should check the rest part of parsed string.

[takei-yuya]

Currently, pfi::lang::lexical_cast don't throw any exception for invalid input which has "valid prefix". (e.g. lexical_cast<int>("0.5") )
But caller have no way to check whole of input is valid? or only prefix is valid?
So lexical_cast should throw exception for such input.

And also we need some tests for cast.h to make the spec clear.