Simplify using struct parametric to change function interfaces and support typedefs.

[hongted]

Currently in order to parameterize the function interface using struct parametrics, something like the following needs to be written:

struct Params {
  bit_width : u32,
  num_elements : u32,
}

pub fn StridedArrayUpdate<params : Params,
                          W : u32 = params.bit_width,
                          N : u32 = params.num_elements >(
  array_in : bits[W][N],
  index : u32,
  stride : u8,
  val : bits[W]) -> bits[W][N] {
  ...
}

This issue is to simplify the usage to something similar to

struct Params {
  bit_width : u32,
  num_elements : u32,
}

pub fn StridedArrayUpdate<params : Params>(
  array_in : bits[params.bit_width][params.num_elements],
  index : u32,
  stride : u8,
  val : bits[params.bit_width]) -> bits[params.bit_width][params.num_elements] {
  ...
}

Even simpler would be to support typedefs

struct Params {
  bit_width : u32,
  num_elements : u32,
  type ElementType = bits[bit_width],
  type ArrayType = ElementType[num_elements],
}

pub fn StridedArrayUpdate<params : Params>(
  array_in : params.ArrayType,
  index : u32,
  stride : u8,
  val : params.ElementType) -> params.ArrayType {
  ...
}