Add early-stage optimization crate (#556)

* Add simple constant folding and folding tests

lib/preopt/src/lib.rs

@@ -0,0 +1,71 @@
+//! Performes early-stage optimizations on Cranelift IR.
+
+#![deny(
+    missing_docs,
+    trivial_numeric_casts,
+    unused_extern_crates,
+    unstable_features
+)]
+#![warn(unused_import_braces)]
+#![cfg_attr(
+    feature = "clippy",
+    plugin(clippy(conf_file = "../../clippy.toml"))
+)]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    allow(new_without_default, new_without_default_derive)
+)]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    warn(
+        float_arithmetic,
+        mut_mut,
+        nonminimal_bool,
+        option_map_unwrap_or,
+        option_map_unwrap_or_else,
+        print_stdout,
+        unicode_not_nfc,
+        use_self
+    )
+)]
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), alloc)]
+
+#[cfg(not(feature = "std"))]
+extern crate alloc;
+
+extern crate cranelift_codegen;
+// extern crate rustc_apfloat;
+
+mod constant_folding;
+
+use cranelift_codegen::{isa::TargetIsa, settings::FlagsOrIsa, CodegenResult, Context};
+
+/// Optimize the function with available optimizations.
+///
+/// Since this can be resource intensive (and code-size inflating),
+/// it is separated from `Context::compile` to allow DCE to remove it
+/// if it's not used.
+pub fn optimize(ctx: &mut Context, isa: &TargetIsa) -> CodegenResult<()> {
+    ctx.verify_if(isa)?;
+    fold_constants(ctx, isa)?;
+
+    Ok(())
+}
+
+/// Fold constants
+pub fn fold_constants<'a, FOI>(ctx: &mut Context, fisa: FOI) -> CodegenResult<()>
+where
+    FOI: Into<FlagsOrIsa<'a>>,
+{
+    constant_folding::fold_constants(&mut ctx.func);
+    ctx.verify_if(fisa)?;
+    Ok(())
+}
+
+/// This replaces `std` in builds with `core`.
+#[cfg(not(feature = "std"))]
+mod std {
+    pub use alloc::{boxed, slice, string, vec};
+    pub use core::*;
+}