Refactor some parsing code

Makes the parse module a bit easier to read. Also makes the struct attr
and field attr parsing methods more similar and prepares for additional
struct attributes.

macro/src/ir.rs

@@ -86,3 +86,14 @@ pub(crate) enum MapKey {
  Float(syn::LitFloat),
  Bool(syn::LitBool),
 }
+
+impl From<MapKey> for Expr {
+ fn from(src: MapKey) -> Self {
+ match src {
+ MapKey::Str(v) => Self::Str(v),
+ MapKey::Int(v) => Self::Int(v),
+ MapKey::Float(v) => Self::Float(v),
+ MapKey::Bool(v) => Self::Bool(v),
+ }
+ }
+}