Make more code work with no_std. (#407)

* Make more code work with no_std.

no_std support is still incomplete, but this patch takes care of the
bulk of the straightforward parts.

fuzz/fuzz_targets/compile.rs

@@ -8,10 +8,10 @@ extern crate wasmparser;
 extern crate wasmtime_environ;
 extern crate wasmtime_jit;
 
+use alloc::rc::Rc;
+use core::cell::RefCell;
 use cranelift_codegen::settings;
-use std::cell::RefCell;
 use std::collections::HashMap;
-use std::rc::Rc;
 use wasmparser::validate;
 use wasmtime_jit::{CompilationStrategy, CompiledModule, Compiler, NullResolver};