[Feature] Eliminate const variables as such

[acoglio]

🚀 Feature

Currently there are three kinds of variables:

1. Constant (const) -- their value is known at compile time and never changes.
2. Immutable (let) -- their value never changes; it may or may not be known at compile time.
3. Mutable (let mut) -- their value may change; it may or may not be known at compile time.
   Constant variables are not allocated in the R1CS circuit, while immutable and mutable variables are.

We could simplify the language to have just immutable (let) and mutable (let mut) variables, without any const variables. The compiler could automatically infer, via constant propagation and folding, which variables have values known at compile time and therefore need not be allocated in the R1CS circuit.

Motivation

This could make the language simpler to learn and use.

This would also simplify type checking/inference, which would not have to worry about constancy (see issue #514).

It could potentially make the translation to R1CS slightly more complicated by having to figure out automatically which variables need not be allocated, but arguably this is something that we may want the compiler to do anyhow, so perhaps there is no downside from this standpoint.

Implementation

This would affect the Pest grammar, the ASTs, type checking/inference, and the translation to R1CS.