feat: Add parsing support for primitive types.

src/ast.rs

@@ -64,6 +64,7 @@ pub enum Statement {
     Let {
         name: Box<str>,
         name_span: Span,
+        type_: Option<Type>,
         value: Option<Expression>,
     },
 
@@ -94,3 +95,11 @@ pub enum Statement {
 
     Expr(Expression),
 }
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Type {
+    Integer { size: u8, signed: bool },
+    Boolean,
+
+    Named { name: Box<str>, name_span: Span },
+}