Improve binding of a `const` value

[jlapeyre]

Currently

const int[32] n = 3;

Produces a Cast expression from a 128 bit wide 3 to int[32]. This loosely follow a document on how Rust treats literals in the first levels of compilation. We recently introduced storing the value of x in a table of const values. We store this Cast expression. This then has to be parsed to extract the value 3 when it is used later.

This Cast should either be avoided, or immediately reduced to a Literal of type int[32]. In particular, the Cast should never appear in the ASG (or any other persistent structure). This does not change the semantics of the program, and simplifies using the table of const values.

This would allow us, for example, to avoid this match arm:

openqasm3_parser/crates/oq3_semantics/src/asg.rs

Lines 169 to 193 in 02a7c6b

	impl TryFrom<&TExpr> for u32 {
	type Error = TryFromU32Error;
	fn try_from(value: &TExpr) -> Result<Self, TryFromU32Error> {
	match &value.expression {
	Expr::Cast(thecast) => {
	match **thecast {
	Cast {
	operand:
	TExpr {
	expression:
	Expr::Literal(Literal::Int(IntLiteral {
	value: int_value,
	sign: true,
	})),
	ty: _, // Type::Int(_, _),
	},
	typ: _, // Type::Int(_, _),
	} => u32::try_from(int_value).map_err(|_| TryFromU32Error),
	_ => Err(TryFromU32Error),
	}
	}
	_ => Err(TryFromU32Error),
	}
	}
	}