Add Lua helper functions

Functions:
`vector.apply(v, func)`
`math.sign(x, limit)`
I find this function very confusing.

Let x = 1 and limit = 5
1 > 5  false
1 < -5  false
returns 0
This does not appear to be what the documentation says it should do

Let x = -1 and limit = -5
-1 < -5? No
-1 < 5? Yes
returns -1
????