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/x86/mod.rs

@@ -10,6 +10,9 @@ pub mod settings;
 #[cfg(feature = "unwind")]
 mod unwind;
 
+#[cfg(feature = "unwind")]
+pub use fde::map_reg;
+
 use super::super::settings as shared_settings;
 #[cfg(feature = "testing_hooks")]
 use crate::binemit::CodeSink;