pass: remove redundant jumps and dangling labels (#81)

In jump-table-like constructs, the natural way of writing the code can sometimes produce redundant jumps or labels. Therefore some basic cleanup steps have been proposed. This diff adds two transforms:

1. Remove unconditional jumps to a label immediately following.
2. Remove labels with no references at all.

Fixes #75

pass/pass.go

@@ -11,6 +11,8 @@ import (
 // Compile pass compiles an avo file. Upon successful completion the avo file
 // may be printed to Go assembly.
 var Compile = Concat(
+	FunctionPass(PruneJumpToFollowingLabel),
+	FunctionPass(PruneDanglingLabels),
 	FunctionPass(LabelTarget),
 	FunctionPass(CFG),
 	FunctionPass(Liveness),