Rename gen to gen_llvm

This way, we have gen_dev and gen_llvm representing the two
different compiler backends.

compiler/gen_llvm/src/lib.rs

@@ -0,0 +1,9 @@
+#![warn(clippy::dbg_macro)]
+// See github.com/rtfeldman/roc/issues/800 for discussion of the large_enum_variant check.
+#![allow(clippy::large_enum_variant)]
+// we actually want to compare against the literal float bits
+#![allow(clippy::clippy::float_cmp)]
+
+pub mod llvm;
+
+pub mod run_roc;