Add support for declaring named constants (scalars, vectors, matrices)

[tohrnii]

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

Constants could be of the following three types:

• scalars
• vectors
• matrices

The constants should be declared in a separate section.

Declaring constants could be done like so:

constants:
  a: 123 // a scalar constant
  b: [1, 2, 3] // a vector constant
  c: [
    [1, 2, 3],
    [4, 5, 6],
  ] // a matrix constant

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

let x = b[1]
let y = c[0][2]

Something like this could also be possible:

let x = c[1][1..3] // this sets x to [5, 6]

Other open suggestions by @grjte and @bobbinth to easily distinguish between trace columns and constants:

• Use CAPITALS for constant names. If we do this, we should decide if we want to show an error to the user or a warning to use correct convention.
• Another option is to prefix trace columns with a some special symbol.

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

• parser
• IR
• codegen

[tohrnii]

We need to add a check for mismatch in the number of columns in rows in a matrix to the IR and throw an error if they are unequal. For eg, the following should throw an error:

A: [[1, 2, 3], [4, 5]]

[tohrnii]

Closed by #63, #94 and #95.