Add bytecode VM backend (compile AST to bytecodes + stack-based VM)

New execution mode: ./run vm <file.j> compiles AST to bytecodes and
runs them in a while/switch loop. Ints/floats live on the stack (no
heap allocation), ~7.7x faster than the tree-walking interpreter.

Implements: opcodes, compiler with backpatching (if/while), stack VM
with arithmetic, comparisons, variables, strings, and print/println.
Reorganizes backend into src/backend/eval/ and src/backend/bytecode/.

projects/while.j

@@ -1,4 +1,4 @@
 x = 0
-while x < 10000000:
+while x < 1000000000:
     x = x + 1
-print x
+println(x)