Replaces the free-standing functions in f32, &c.

The free-standing functions in f32, f64, i8, i16, i32, i64, u8, u16,
u32, u64, float, int, and uint are replaced with generic functions in
num instead.

If you were previously using any of those functions, just replace them
with the corresponding function with the same name in num.

Note: If you were using a function that corresponds to an operator, use
the operator instead.
Forgot to grep for a function
And I forgot to run the benchmarks after rebasing
I forgot the changes to the docs as well

Apparently yesterday wasn't my day, and I forgot to add the changes to
all the tests apparently, and in the end forgot the docs extra much.
Please documentation, forgive me, I really do love you, I hope you
forgive me.

Next time we'll meet tutorial, I promise to bring cookies and tea. I
really want to be best-friends-forever with you, <3.

XOXO
auto merge of #7117 : jensnockert/rust/freestanding, r=cmr

The free-standing functions in f32, f64, i8, i16, i32, i64, u8, u16,
u32, u64, float, int, and uint are replaced with generic functions in
num instead.

This means that instead of having to know everywhere what the type is, like

~~~
f64::sin(x)
~~~

You can simply write code that uses the type-generic versions in num instead, this works for all types that implement the corresponding trait in num.

~~~
num::sin(x)
~~~

Note 1: If you were previously using any of those functions, just replace them
with the corresponding function with the same name in num.

Note 2: If you were using a function that corresponds to an operator, use the
operator instead.

Note 3: This is just https://github.com/mozilla/rust/pull/7090 reopened against master.