This makes defining guards easier, since it does the boilerplate
for you and raises at compile time in the defguard macro when
you do something not allowed in guards (as opposed to whenever
the new guard happens to be used as a guard).
It takes a `defguard name(args) when implementation` invocation,
passes the implementation through `:elixir_expand.expand/2` in
a guard context, and if nothing is raised, transforms the impl
into a macro that works both inside and outside of guard contexts.
Signed-off-by: José Valim <jose.valim@plataformatec.com.br>
Chris Keele
c2eee76805