Parameterize program solving on a FunctionKind

This new flag determines whether we should introduce a new kind to
represent lambda sets, or whether lambdas should be erased. The latter
is not yet implemented.

crates/compiler/solve/src/kinds.rs

@@ -0,0 +1,8 @@
+/// How function kinds should be represented in the type system.
+#[derive(Debug)]
+pub enum FunctionKind {
+    /// Function values are solved to lambda sets; lambda sets are the kind.
+    LambdaSet,
+    /// Function values are erased, no kind is introduced.
+    Erased,
+}