Introduce is_map_key/2 guard BIF

This complements the `map_get/2` guard BIF introduced in #1784.

Rationale.

`map_get/2` allows accessing map fields in guards, but it might be
problematic in more complex guard expressions, for example:

foo(X) when map_get(a, X) =:= 1 or is_list(X) -> ...

The `is_list/1` part of the guard could never succeed since the
`map_get/2` guard would fail the whole guard expression. In this
situation, this could be solved by using `;` instead of `or` to separate
the guards, but it is not possible in every case.

To solve this situation, this PR proposes a `is_map_key/2` guard that
allows to check if a map has key inside a guard before trying to access
that key. When combined with `is_map/1` this allows to construct a
purely boolean guard expression testing a value of a key in a map.

Implementation.

Given the use case motivating the introduction of this function, the PR
contains compiler optimisations that produce optimial code for the
following guard expression:

foo(X) when is_map(X) and is_map_key(a, X) and map_get(a, X) =:= 1 -> ok;
foo(_) -> error.

Given all three tests share the failure label, the `is_map_key/2` and
`is_map/2` tests are optimised away.

As with `map_get/2` the `is_map_key/2` BIF is allowed in match specs.

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