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/module.rs

@@ -2,6 +2,7 @@ use crate::context::layout_vmcontext;
 use crate::data_segment::{declare_data_segment, emit_data_segment};
 use crate::function::{declare_functions, emit_functions};
 use crate::table::{declare_table, emit_table};
+use alloc::string::String;
 use cranelift_codegen::isa::TargetFrontendConfig;
 use faerie::{Artifact, Decl, Link};
 use wasmtime_environ::{Compilation, DataInitializer, Module, Relocations};