The SQL standard defines a function called TRIM_ARRAY that surprisingly has syntax that looks like a function! So I implemented it using a thin wrapper around our array slice syntax. It is literally just ($1)[1:$2].
An interesting case that I decided to handle by explaining it in the docs is that this won't give you the first n elements if your lower bound is not 1. My justification for this is 1) non-standard lower bounds are so rare in the wild that 2) people using them can just not use this function. The alternative is to go through the unnest dance (or write it in C) which defeats inlining.
I don't recall ever seeing non-default lower bounds, so I actually think it's OK to just rule out that scenario, but why not something like this:
($1)[:array_lower ($1, 1) + $2 - 1]
Note that I've used the 9.6 feature that allows omitting the lower bound.