Add epsilon to Basics

[TheSeamau5]

Defining an epsilon value is pretty basic need when dealing with arithmetic, especially to define an approximately equal operation (due to floating point errors).

While this can perfectly live in a library, I think it's worth being in the core library in the Basics module.

In JS, Number.EPSILON is defined as 2.2204460492503130808472633361816E-16, or 2-52.
This is not well supported accross browsers, as such, it's worth defining it as a constant directly.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/EPSILON

[evancz]

Consolidated all the math related stuff into the #721 meta issue. Follow along there!

[evancz]

Consolidated all the math related stuff into the #721 meta issue. Follow along there!