namespace sts::ams -> ams::exosphere, ams::.

This is to facilitate future use of ams:: namespace code in
mesosphere, as we'll want to include ams::util, ams::result, ams::svc...

This implements waitable management for Events (and
implements Events). It also refactors PM to use new
Event/Waitable semantics, and also adds STS_ASSERT
as a macro for asserting a boolean expression. The
rest of stratosphere has been refactored to use
STS_ASSERT whenever possible.