[MICRO]: fn: add uncons, unsnoc and its component projections

[ProofOfKeags]

Change Description

This adds a handful of useful fundamental slice projections. It is motivated by this discussion. I'm not attached to the names but they are not arbitrary:

uncons, unsnoc, head, tail, init, last

Steps to Test

make unit pkg=fn

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[Roasbeef]

LGTM 🦕

[ziggie1984]

Cool new features 👌