Returned libm, support std

[arduano]

Removed the copied libm code, instead adding libm as a dependency again.

Also, added the no_std feature to enable the use of libm, otherwise normal rust std is used.

[arduano]

The original rationale for libm being included in this repository is explained by this comment:

//! libm in pure Rust
//!
//! This is pulled from  https://github.com/rust-lang-nursery/libm
//! Maintainers of that project have gone dark, and I had to fix some things
//! so I just pulled it into simdeez.
//! This is all just to support no_std
//!

libm on crates.io appears to be active again with recent updates.