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-obj/src/context.rs

@@ -1,9 +1,11 @@
+use alloc::boxed::Box;
+use alloc::vec::Vec;
+use core::ptr;
 use cranelift_codegen::isa::TargetFrontendConfig;
 use cranelift_entity::EntityRef;
 use cranelift_wasm::GlobalInit;
 use std::collections::hash_map::Entry;
 use std::collections::HashMap;
-use std::ptr;
 use wasmtime_environ::{Module, TargetSharedSignatureIndex, VMOffsets};
 
 pub struct TableRelocation {
@@ -28,7 +30,7 @@ pub fn layout_vmcontext(
         let target_index = match signature_registry.entry(sig) {
             Entry::Occupied(o) => *o.get(),
             Entry::Vacant(v) => {
-                assert!(signature_registry_len <= ::std::u32::MAX as usize);
+                assert!(signature_registry_len <= ::core::u32::MAX as usize);
                 let id = TargetSharedSignatureIndex::new(signature_registry_len as u32);
                 signature_registry_len += 1;
                 *v.insert(id)