tests: test functions with random signatures (#216)

Adds a test for function signature memory layout by generating functions with
random signatures. This confirms that the size and offsets computed by
`gotypes` agree with `asmdecl`.

Updates #191 #195

tests/signature/gen.go

@@ -0,0 +1,4 @@
+// Package signature tests handling of random function signatures.
+package signature
+
+//go:generate go run asm.go -out signature.s -stubs stub.go -seed 42 -num 256