[m-users.] deleting the Erlang backend

[Peter Wang]

Hi,

The Erlang backend has been deprecated as of the Mercury 20.01 release.
If anyone happens to be using the Erlang backend, please let us know
before we commit to deleting it for good.

Rationale: the Erlang backend was an experiment that turned out to
not be useful practically, and now is an ongoing maintainance burden.
For example, when adding or modifying parts of the standard library
implemented in foreign code, it's often necessary to add or modify an
Erlang implementation. This can be tricky as none of the Mercury
developers are invested in the Erlang language or ecosystem.
It can also be tricky for features that don't map cleanly into Erlang.
An example is the fixed width integer types: since Erlang only has
arbitrary precision integers, the behaviour of fixed width integers
would need to be emulated (currently they are not).

Peter