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-api/src/values.rs

@@ -2,7 +2,7 @@ use crate::externals::Func;
 use crate::r#ref::{AnyRef, HostRef};
 use crate::runtime::Store;
 use crate::types::ValType;
-use std::ptr;
+use core::ptr;
 
 use cranelift_codegen::ir;
 use wasmtime_jit::RuntimeValue;