encapsulate-main branch

[Peter Lobsinger]

The encapsulate-main branch moves tracking of :main-tagged subs from
the subs themselves to packfiles (which should really be taking care
of this kind of thing). At the same time, it tackles TT #1704, which
requires a :main sub (no more automagic
first-encountered-sub-is-main).

At this point, it passes most tests (a few examples need some work).
Some HLLs pass (winxed, lua with :main tags added to PIR testsuite).

NQP-rx needs some work to generate :main subs. I have some ideas about
how to "fix" it, but they are hackish and of poor quality. If someone
familiar with this language could make the appropriate changes, this
would be much appreciated.