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/environ/src/data_structures.rs

@@ -14,6 +14,9 @@ pub mod settings {
 
 pub mod isa {
     pub use cranelift_codegen::isa::{CallConv, RegUnit, TargetFrontendConfig, TargetIsa};
+    pub mod fde {
+        pub use cranelift_codegen::isa::fde::map_reg;
+    }
 }
 
 pub mod entity {