[mypyc] Make tuple packing and unpacking more efficient

[JukkaL]

Previously returning a tuple from a function resulted in redundant increfs and decrefs
for each item, and similarly unpacking the returned tuple in an assignment had extra
incref/decref pair per item. This PR introduces these changes to make this better:

• Creating a tuple steals the items always.
• Accessing a tuple item optionally borrows the item.
• A borrowed reference can be turned into a regular one using the new Unborrow op.
• The no-op KeepAlive op can steal the operands to avoid decrefing the operands.

Assignment from tuple now uses the three final features to avoid increfs and decrefs
when unpacking a tuple in assignment. The docstrings in this PR contain additional
explanation of how this works.

In a micro-benchmark this improved performance by about 2-5%. In realistic examples
the impact is likely small, but every little helps.

Here is an example where this helps:

def f() -> tuple[C, C]:
    return C(), C()  # Avoid 2 increfs and 2 decrefs

def g() -> None:
    x, y = f()  # Avoid 2 increfs and 2 decrefs
    ...

[AlexWaygood]

One or two typos I spotted:

[JukkaL]

I'm not sure what's going on with the build failures. The tests seem to pass locally, but in CI the mypyc-compiled builds get a sigterm, apparently.