Implement additional PUSH_* bytecode instructions

Implement the following bytecode instructions:

  * PUSH_UNDEFINED
  * PUSH_NULL
  * PUSH_FAILED
  * PUSH_EMPTY_ARRAY

These instructions push simple JavaSccript values to the stack directly,
without going through constants. This makes the bytecode slightly
shorter and the bytecode generator somewhat simpler.

Also note that PUSH_EMPTY_ARRAY allows us to avoid a hack which protects
the [] constant from modification.
Improve semantics of the TEXT bytecode instruction

The TEXT instruction now replaces position at the top of the stack with
the input from that position until the current position. This is simpler
and cleaner semantics than the previous one, where TEXT also popped an
additional value from the stack and kept the position there.
Regenerate lib/parser.js after bytecode changes