Add support for overloaded methods / functions

[davesmith00000]

This is valid GLSL, and Scala for that matter.

def mod289(x: vec3): vec3 =
  x - (floor(x * (1.0f / 289.0f)) * 289.0f)

def mod289(x: vec2): vec2 =
  x - (floor(x * (1.0f / 289.0f)) * 289.0f)

But Ultraviolet complains that the function is already declared:

mod289 is already defined as method mod289

Note that overloaded methods must all be defined in the same group of toplevel definitions