Update call argument storing in the reverse mode

[PetroZarytskyi]

This PR moves the storing system of ref-type call arguments to the same framework as the rest of Clad. This leads to 2 changes in the produced code:

1. Only original variables (not temporaries _t) are used as pullback args

// forward pass
_t0 = a;
modify(_t0); // a is modified so we store it
...
// reverse pass
a = _t0;
// modify_pullback(_t0, ...) <-- used to be this
modify_pullback(a, ...);

This is safe because pullbacks have the same storing system as the gradients, meaning they are responsible for restoring a to its original state.

2. In loops, the tape is handled a little differently:

a = clad::back(_t0);
modify_pullback(a, ...);
clad::pop(_t0);

became

a = clad::pop(_t0);
modify_pullback(a, ...);

This PR also updates the documentation of this part.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[vgvassilev]

Lgtm!