Add support for intermediate variables (scalars, vectors, matrices)

[tohrnii]

We should add support for variables. The syntax was defined by @bobbinth in the original Constraints Description Language discussion here. Taken from @bobbinth's original post:

A variable is defined using a let keyword. For e.g., let a = b * c

Variables could be of the following three types:

• scalars
• vectors
• matrices

Declaring variables could be done like so:

trace_columns:
  main: [a, b, c]
transition_constraints:
  let x = a + 123 // a scalar variable
  let y = [a + 1, b + 2, c + 3] // a vector variable
  let z = [
    [a + 1, b + 2, c + 3],
    [a + 4, b + 5, c + 6],
  ] // a matrix variable

Referring to vector variables inside expressions could be done with index notation:

let a = [m, n]
let x = a[0] + 1

We could build variables from column references as well:

trace_columns:
  main: [a, b]

let x = [a, b]   // x is a vector with current values from a and b
let y = [a', b'] // y is a vector with next values from a and b

Variables should only be defined in boundary_constraints and transition_constraints sections.

This task involves making changes to the parser, IR and codegen.

• parser
• IR
• codegen

[tohrnii]

Variable support is remaining for boundary constraints. The issue has been moved to #106.

Closed by #100.