Pre-allocate rpn_value stack objects

[mcspr]

Right now stack values are always created in a brand new memory location, requiring call to malloc()
(and are tracked via shared_ptr, to provide a way to detect going out of scope / simulate gc)

Perhaps, it could either:

• pre-allocate chunk of memory to store rpn_value, avoiding malloc() calls each time, providing real 'stack'-like behaviour.
• (slightly easier?) pre-allocate rpn_value objects and track which ones are 'free' to use