Update Float

[sampersand]

This updates Float:

• Move constants inside it
• Added Numeric::_XXX interfaces for operators, as well as Numeric::_Coerce
• Float#{+,-,*,/,%,**,<,>,<=,>=,<=>,divmod,quo}: Now uses Numeric::_XXX methods.
• Multiple methods (eg Float#=== and Float#fdiv) are now aliases
• Revamped Float#round.
• Moved all functions not defined on Float, but rather Numeric, to the bottom of the function and marked them as deprecated
• Overhauled the tests

[soutaro]

The changes look good, but it doesn't work well with the current version of Steep. It is a kind of bug of Steep, so I will fix the problem and then merge this PR, to avoid unnecessary type checking issues at the end users side.