Report diagnostics in Type::from_ast

[pzipper]

Currently, Type::from_ast looks like this:

from (src/types.rs):

/// Attempts to resolve a constant type value from the provided expression.
pub fn from_ast(
    cx: &mut Context,
    unit: &mut Unit,
    scope: &Scope,
    expr: &ast::Expr,
) -> Result<Self, ()> {
    let Value::Type(final_ty) = Value::eval(
        IntermediateExpr::verify(cx, unit, scope, expr)?
            // verify that the value is a type
            .coerce(&Type::Type)
            .expect("TODO: report non-type in type position"),
    )
    .expect("TODO: report non-constant type")
    else { 
        unreachable!("value should be of type `type` as verified above")
    };

    Ok(final_ty)
}

Rather than expecting and panicking when a value is used as a type, we should report a diagnostic. Ideally, this diagnostic would be a type-mismatch diagnostic which may be re-used in other situations, such as variable type mismatches, etc.