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`

crates/debug/src/frame.rs

@@ -1,6 +1,6 @@
-use crate::transform::map_reg;
 use std::collections::HashMap;
 use wasmtime_environ::entity::EntityRef;
+use wasmtime_environ::isa::fde::map_reg;
 use wasmtime_environ::isa::{CallConv, TargetIsa};
 use wasmtime_environ::wasm::DefinedFuncIndex;
 use wasmtime_environ::{FrameLayoutChange, FrameLayouts};