Implement return

[jlapeyre]

Breaking recursion in enum Expr done differently here

Note that in enum Expr in asg.rs, variants before this PR have looked like this:

openqasm3_parser/crates/oq3_semantics/src/asg.rs

Lines 118 to 121 in b174b2b

	pub enum Expr {
	ArraySlice(ArraySlice),
	BinaryExpr(BinaryExpr),
	UnaryExpr(UnaryExpr),

For return expressions we introduce a variant Return(Box<ReturnExpression>). This probably means that consumers have to treat Return differently because the indirection happens in a different place (in the variant). The other choice would be to continue with the previous pattern of putting the Box in the struct ReturnExpression.

It probably makes sense to choose one way and do it uniformly.

Change in fn from_expr in syntax_to_semantics.rs

Before the expression from the AST was unwrapped and passed to from_expr. This is changed in this PR. Now we pass the Option to from_expr. This simplifies much code in syntax_to_semantics.rs (as is evident by looking at this PR). panics due to unwrap will be moved to different places in the code. Of course, eventually, we need to avoid panic ing altogether.

Note that there is a check for return in global scope. But we need something more systematic than this

• See Organize errors for constructs not allowed in a scope #136

Closes #133