ask ASM to compute stack maps for us

this is necessary now that we're on Java 8. since Java 7 it's
required to include the stack map info.

it's a bit tempting to use a V1_5 ClassWriter and hope nothing breaks,
but in this repo we're now compiling everything on Java 8, so it seems
entirely possible that MethodRipper might read some Java 8 specific
bytecode out of a method and then inline it into a generated class. so
better we be safe.