Un-deprecate Array Append

[kentosugama]

Array append is a useful function, and so should not be deprecated.

Documentation still includes warning of its linear complexity and suggestions to use a buffer instead.

[rossberg]

Drive-by comment: Being useful is not a sufficient condition for supporting a feature, almost every imaginable feature is useful in certain situations, yet most are counter-productive in the grand scheme of things. This one likely is unknowingly misused 95% of the time, no matter what the docs will say.

[crusso]

Maybe we could use the unicode :warning: symbol to silence warnings on identifiers. Only half joking.

[ByronBecker]

Maybe keep append deprecated, but just rename it to concat which imo is a better name for the joining of two arrays (most people won't use it to append just a single item).

[ggreif]

#630 did this. Closing.