Revert "add support for named exception handlers for use in labeled loops"

This feature gets moved into branch 'loop_labels'.
Add MVMNull REPR, instance->VMNull.
Add static inline func for doing a VM-null check.
Make nqp::null and nqp::isnull VM-null aware.
Teach serialization about VM nulls.
Teach P6opaque about VMNull; eliminate ass_null.
Teach ifnonnull about VMNull.
Update continuations code for VMNull.
Update invocation handling for VMNull.
Factor VMNull into null HLL mapping.
Make array and hash use VMNulls; fix fallout.

With this, NQP still builds/passes tests.
Use MVM_is_null in gethllsym.
Use MVM_is_null in sp_findmeth.
A couple more placs MVM_is_null is needed.

With this, Rakudo builds and sanity-tests again.
Do VM null check in event loop.
Lazily allocate lexical containers.

This means that most of the time we won't allocate $/ and $!, and
perhaps $_ too, when they are never used.
Optimize frame allocation.

Use calloc rather than malloc + memset.
Remove unrequired memory barriering.

We didn't expose the frame anywhere else at the point we set these, so
no need to barrier.
Remove BOM-discarding code from MVM_string_utf8_decode()

This code is unreachable on a platform where char is signed, because there
utf8[0] will be between -128 and and 127 inclusive, and hence never equal to
0xEF. (ie x86, x86_64). On a platform where char is unsigned, the code is
reached, and it kills the Rakudo build. Specifically, this happens because
it munges strings being deserialised, with the particular problem string being
the character U+FEFF, which is used in pre-compiled NQP code. Deserialising
what should be a 1 char string U+FEFF as a 0 char string causes a very strange
"Could not locate compile-time value for symbol Whatever" error when
compiling the Perl 6 setting. Oh the strange side effects of bugs...

If BOM-nomming is needed, it should be in I/O specific code, and only at the
start of files, not in the general-purpose NFG conversion routines.
MVM_string_utf8_decode() should take a MVMuint8 *, not a char *

Conceptually, UTF-8 is sequences of Octets, which are unsigned, so use an
unsigned type.