Add base structure for dev compiler backend

Currently we only generate a single static function, but it is probably
loaded and run with jit. This is the base to start generating proper
code.

compiler/gen_dev/src/lib.rs

@@ -0,0 +1,65 @@
+#![warn(clippy::all, clippy::dbg_macro)]
+// I'm skeptical that clippy:large_enum_variant is a good lint to have globally enabled.
+//
+// It warns about a performance problem where the only quick remediation is
+// to allocate more on the heap, which has lots of tradeoffs - including making it
+// long-term unclear which allocations *need* to happen for compilation's sake
+// (e.g. recursive structures) versus those which were only added to appease clippy.
+//
+// Effectively optimizing data struture memory layout isn't a quick fix,
+// and encouraging shortcuts here creates bad incentives. I would rather temporarily
+// re-enable this when working on performance optimizations than have it block PRs.
+#![allow(clippy::large_enum_variant)]
+
+use bumpalo::Bump;
+use object::write::Object;
+use roc_collections::all::{MutMap, MutSet};
+use roc_module::symbol::{Interns, Symbol};
+use roc_mono::ir::Proc;
+use roc_mono::layout::Layout;
+use target_lexicon::{BinaryFormat, Triple};
+
+pub mod elf;
+pub mod run_roc;
+pub mod x86_64;
+
+pub struct Env<'a> {
+    pub arena: &'a Bump,
+    pub interns: Interns,
+    pub exposed_to_host: MutSet<Symbol>,
+}
+
+/// build_module is the high level builder/delegator.
+/// It takes the request to build a module and output the object file for the module.
+pub fn build_module<'a>(
+    env: &'a Env,
+    target: &Triple,
+    procedures: MutMap<(Symbol, Layout<'a>), Proc<'a>>,
+) -> Result<Object, String> {
+    match target.binary_format {
+        BinaryFormat::Elf => elf::build_module(env, target, procedures),
+        x => Err(format! {
+        "the binary format, {:?}, is not yet implemented",
+        x}),
+    }
+}
+
+trait Backend {
+    /// new creates a new backend that will output to the specific Object.
+    fn new() -> Self;
+
+    /// reset resets any registers or other values that may be occupied at the end of a procedure.
+    fn reset(&mut self);
+
+    /// build_proc creates a procedure and outputs it to the wrapped object writer.
+    /// This will need to return the list of relocations because they have to be added differently based on file format.
+    /// Also, assembly will of course be generated by individual calls on backend like may setup_stack.
+    fn build_proc<'a>(&mut self, env: &'a Env, _proc: Proc<'a>) -> &'a [u8] {
+        let mut out = bumpalo::vec![in env.arena; 0x55, 0x48, 0x89, 0xE5];
+        let body = [0xb8, 0x06, 0x00, 0x00, 0x00];
+        out.extend(body.iter());
+        out.push(0x5D);
+        out.push(0xC3);
+        out.into_bump_slice()
+    }
+}