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-debug/src/write_debuginfo.rs

@@ -1,9 +1,11 @@
+use alloc::string::String;
+use alloc::vec::Vec;
 use gimli::write::{Address, Dwarf, EndianVec, Result, Sections, Writer};
 use gimli::{RunTimeEndian, SectionId};
 
+use core::result;
 use faerie::artifact::{Decl, SectionKind};
 use faerie::*;
-use std::result;
 
 #[derive(Clone)]
 struct DebugReloc {