Avoid generating MutableSeq in invokes

[ice1000]

In the JIT-compiled defs.

Right now we generate these MutableSeq to store some local variables of type Term:
https://gist.github.com/ice1000/70fd92ce92bd744c663e4f7e81d61192#file-_baka-java-L529

But they can be replaced with stack variables.

They were created to hack javac, who insist on "captured variables must be final or effectively final", while these local vars are re-assigned in the pattern matching code, so javac rejects them. But this is not a limitation of the JVM itself, and since we're generating bytecode directly, we should flatten these MutableSeq onto the stack.