Support constant attribute for function variable

[katat]

We may need to instruct if a variable is a constant during assignment, as the assigned value can't represent itself as a constant at type checking phase until synthesizer phase in some cases.

For example:

// this value being assigned should be a constant
let size_bit_len = bits::bit_len(SIZE);

But the return signature bits::bit_len can't reflect it is a constant:

fn bit_len(const SIZE: Field) -> Field;

To instruct the size_bit_len to be a constant field in order to bypass certain type checks, we would:

const size_bit_len = bits::bit_len(SIZE);

This would bypass the checks in the case:

comparator::less_than(size_bit_len, ii, 2)

where the less_than signature is

fn less_than(const LEN: Field, lhs: Field, rhs: Field) -> Field

At synthesizer phase, the assignment can check if the value being assigned is a constant based on the ConstOrVar struct.

[katat]

Maybe the better way would delay this type check to the MAST phase where it can propagate the constant values.
So it can just check if the argument being propagated is a constant.

To do so, we will need to propagate constant values from both builtins and native functions.