• Improve performance.
• Bug fix.

Change the structure of execution stack and context radically.

• Local variables and information necessary for execution are now placed on the execution stack.
• The execution stack is handled by a raw pointer, not by index.
• Instruction sequence is also scanned by a raw pointer.

Bug fixes.

1. Fix problem that Bignum objects created from number literals are incorrectly freed by GC.
2. Fix bug about a parsing priority of unary minus and exponential expressions.