Implement global math functions

[sjbarag]

The BrightScript language supports 17 global math functions:

• Abs(x as Float) as Float
• Atn(x as Float) as Float
• Cdbl(x as Integer) as Float
• Cint(x as Float) as Integer
• Cos(x as Float) as Float
• Csng(x as Integer) as Float
• Exp(x as Float) as Float
• Fix(x as Float) as Integer
• Int(x as Float) as Integer
• Log(x as Float) as Float
• Rnd(range as Integer) as Integer
• Rnd(0) as Float
• Sgn(x as Float) as Integer
• Sgn(x as Integer) as Integer
• Sin(x as Float) as Float
• Sqr(x as Float) as Float
• Tan(x as Float) as Float

We should get those implemented! Luckily they seem to all be wrappers around JavaScript's Math native functions, so these should be relatively easy to implement as native functions. Also luckily: they all operate on either Float or Int32, so we shouldn't have to worry about dealing with 64-bit integer math!

[sjbarag]

I've marked this as "Good first issue" because we have a pretty good pattern around global functions now. Also note that -- because this is a first issue -- a first-time contributor doesn't have to implement all of these at once. That's a pretty big undertaking. We can always split this into multiple issues to handle families of math functions (e.g. "all the trigonometry functions", etc.)

[sjbarag]

Closing in favor of https://github.com/sjbarag/brs/milestone/3 now that I've split this out