Multiple fixes and added features

[mikem8891]

In fn to_integer, I changed it to skip some extra steps. The current code for to_integer calls trunc which calculates the integer then wraps it in a Ratio<T> then to_integer unwraps the numer that trunc returned.

Added fn floor_to_int,ceil_to_int and round_to_int for #149. These are slightly simpler than the original versions and allow getting to the integer without the value being wrapped in a Ratio by the function and unwrapped by the user. Slight changes fix #143.

fn floor, ceil, and round were changed to use the *_to_int functions.

Added a remainder function and changed fract to use remainder

Added mixed to return &self as a mixed fraction (T, Ratio<T>)

In Inv::inv, I changed recip to into_recip to prevent a clone operation. This is in the spirit of #110.

Fixes #143 and closes #149 .

This PR supersedes #144 , #145 , #148 .