Remove duplication of map_reg; fixes #1245

Both cranelift-codegen and wasmtime-debug need to map Cranelift registers to Gimli registers. Previously both crates had an almost-identical `map_reg` implementation. This change:
 - removes the wasmtime-debug implementation
 - improves the cranelift-codegen implementation with custom errors
 - exposes map_reg in `cranelift_codegen::isa::fde::map_reg` and subsequently `wasmtime_environ::isa::fde::map_reg`

cranelift/codegen/src/isa/mod.rs

@@ -74,6 +74,12 @@ mod riscv;
 #[cfg(feature = "x86")]
 mod x86;
 
+#[cfg(all(feature = "x86", feature = "unwind"))]
+/// Expose the register-mapping functionality necessary for exception handling, debug, etc.
+pub mod fde {
+    pub use super::x86::map_reg;
+}
+
 #[cfg(feature = "arm32")]
 mod arm32;