very slightly simplify GEFs instruction finding api

[theguy147]

very slightly simplify GEFs instruction finding api

Description/Motivation/Screenshots

The "instruction finding api" of GEF is currently a little confusing. For example gef_get_instruction_at() and gef_current_instruction do basically the exact same thing (the only difference underneath being a generator used vs a list).
Furthermore some of those function names did not properly reflect the code: e.g. gef_current_instruction() does not return the current instruction but the instruction at a specified address.

How Has This Been Tested?

Architecture	Yes/No	Comments
x86-32	✔️
x86-64	✔️
ARM	✖️
AARCH64	✖️
MIPS	✖️
POWERPC	✖️
SPARC	✖️
RISC-V	✖️
make test	✔️

Checklist

• My PR was done against the dev branch, not master.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

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[stale]

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