Refactor unwind generation in Cranelift.

This commit makes the following changes to unwind information generation in
Cranelift:

* Remove frame layout change implementation in favor of processing the prologue
  and epilogue instructions when unwind information is requested.  This also
  means this work is no longer performed for Windows, which didn't utilize it.
  It also helps simplify the prologue and epilogue generation code.

* Remove the unwind sink implementation that required each unwind information
  to be represented in final form. For FDEs, this meant writing a
  complete frame table per function, which wastes 20 bytes or so for each
  function with duplicate CIEs.  This also enables Cranelift users to collect the
  unwind information and write it as a single frame table.

* For System V calling convention, the unwind information is no longer stored
  in code memory (it's only a requirement for Windows ABI to do so).  This allows
  for more compact code memory for modules with a lot of functions.

* Deletes some duplicate code relating to frame table generation.  Users can
  now simply use gimli to create a frame table from each function's unwind
  information.

Fixes #1181.

cranelift/codegen/src/ir/mod.rs

@@ -6,7 +6,6 @@ pub mod dfg;
 pub mod entities;
 mod extfunc;
 mod extname;
-mod framelayout;
 pub mod function;
 mod globalvalue;
 mod heap;
@@ -40,7 +39,6 @@ pub use crate::ir::extfunc::{
     AbiParam, ArgumentExtension, ArgumentPurpose, ExtFuncData, Signature,
 };
 pub use crate::ir::extname::ExternalName;
-pub use crate::ir::framelayout::{FrameLayout, FrameLayoutChange, FrameLayoutChanges};
 pub use crate::ir::function::{DisplayFunctionAnnotations, Function};
 pub use crate::ir::globalvalue::GlobalValueData;
 pub use crate::ir::heap::{HeapData, HeapStyle};