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-wast/src/lib.rs

@@ -22,6 +22,7 @@
     )
 )]
 
+extern crate alloc;
 #[macro_use]
 extern crate failure_derive;
 

wasmtime-wast/src/spectest.rs

@@ -1,10 +1,10 @@
+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, Global, GlobalInit, Memory, Table, TableElementType};
-use std::cell::RefCell;
-use std::collections::HashMap;
-use std::rc::Rc;
+use std::collections::hash_map::HashMap;
 use target_lexicon::HOST;
 use wasmtime_environ::{translate_signature, Export, MemoryPlan, Module, TablePlan};
 use wasmtime_jit::target_tunables;