Add FuncEnvironment hooks to generate prologue and epilogue code.

In some cases, it is useful to do some work at entry to or exit from a
Cranelift function translated from WebAssembly. This PR adds two
optional methods to the `FuncEnvironment` trait to do just this,
analogous to the pre/post-hooks on operators that already exist.

This PR also includes a drive-by compilation fix due to the latest
nightly wherein `.is_empty()` on a `Range` ambiguously refers to either
the `Range` impl or the `ExactSizeIterator` impl and can't resolve.

cranelift/codegen/src/isa/aarch64/inst/unwind.rs

@@ -104,7 +104,7 @@ impl UnwindInfoGenerator<Inst> for AArch64UnwindInfo {
 
         // TODO epilogues
 
-        let prologue_size = if context.prologue.is_empty() {
+        let prologue_size = if context.prologue.len() == 0 {
             0
         } else {
             context.insts_layout[context.prologue.end as usize - 1]