lang: program constants defined as constant expressions are stored literally in IDL

[cavemanloverboy]

if one defines some

#[constant]
/// This is a useful constant
pub const USEFUL_CONSTANT: usize = 2_usize.pow(8);

then the idl will store "2_usize.pow(8)" literally instead of "256".

If there is an easy way to evaluate const expressions before storing it in the IDL, I personally think that is a better outcome. This would allow for easy use of parseInt or similar fn's in typescript.

[acheroncrypto]

Fixed by #2824.