Add .get method.
add .gitignore
Avoid the need to create the dummy for an empty vector.

This makes creation much cheaper (no allocations required).
Fix an incorrect bounds check in .get.

The check was missing the case when index == stable_vec.len() since the
underlying Vec has length that is 1 larger.
Note some issues.
Make reserve* check if the vector moved in memory.

Without doing this, the base_ptr's are not valid, possibly leading to
crashes and segfaults.
Make .extend correct-er.

The pass-in iterator could hold a handle to the vector, so we need to
keep the vector valid whenever we call into it, i.e. the dummy has to
exist for every tick of the loop.
Add tiny methods for retrieving a Handle's StableVec.

Makes for nicer code.