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.

wasmtime-wasi/src/instantiate.rs

@@ -1,12 +1,12 @@
 use super::syscalls;
+use alloc::rc::Rc;
+use core::cell::RefCell;
 use cranelift_codegen::ir::types;
 use cranelift_codegen::{ir, isa};
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::DefinedFuncIndex;
-use std::cell::RefCell;
 use std::collections::HashMap;
 use std::fs::File;
-use std::rc::Rc;
 use target_lexicon::HOST;
 use wasi_common::WasiCtxBuilder;
 use wasmtime_environ::{translate_signature, Export, Module};

wasmtime-wasi/src/lib.rs

@@ -1,3 +1,5 @@
+extern crate alloc;
+
 mod instantiate;
 mod syscalls;