Add refinement types to VeLa

[dbenn]

Refinement types ala Wadler's complement to blame, e.g. f(n:real{n>= 0})

[dbenn]

One obvious use case is time parameters.

[dbenn]

A variable could have such constraints added as well, e.g.

phase:real{phase > 0 and phase < 1} <- 42

although the syntax would require some thought. The code above would give an error, as would the following on the second line:

phase:real{phase > 0 and phase < 1} <- 0.1
phase <- 42