Move constrain and its deps into their own crates

compiler/parse/src/pattern.rs

@@ -0,0 +1,11 @@
+/// Different patterns are supported in different circumstances.
+/// For example, when branches can pattern match on number literals, but
+/// assignments and function args can't. Underscore is supported in function
+/// arg patterns and in when branch patterns, but not in assignments.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum PatternType {
+    TopLevelDef,
+    DefExpr,
+    FunctionArg,
+    WhenBranch,
+}