TRIM_ARRAY

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.

Patch attached.
-- 
Vik Fearing