feat: support function inlining in ZkC

[DavePearce]

(see also #1630)

Summary

The goal of this feature is to support function inlining in ZkC directed by an @inline attribute.

Minimal Example

fn f(x:u32) -> (y:u32) {
   y = 2 * g(x)
}

@inline
fn g(x:u32) -> (y:u32) {
   y = x + 1
}

Then, the compiler is responsible for automatically converting the above into the following form:

n f(x:u32) -> (y:u32) {
   y = 2 * (x + 1)
}

As usual, the goal of this optimisation is to reduce the overhead of function's. In ZkC these can be more expensive than in a generate compute setting, since functions correspond with modules in the final AIR constraints.