* Fixes

CC backend: goto, sort with non-default comparison. last for non-loop blocks.
Version checking (done)
improve XSUB handling (both static and dynamic)
sv_magic can do SvREFCNT_inc(obj) which messes up precalculated refcounts
allocation of XPV[INAHC]V structures needs fixing: Perl tries to free
  them, whereas the compiler expects them to be linked to a xpv[inahc]v_root
list the same as X[IPR]V structures.
ref counts
perl_parse replacement
fix cstring for long strings
compile-time initialisation of AvARRAYs
signed/unsigned problems with NV (and IV?) initialisation and elsewhere?
CvOUTSIDE for ordinary subs
DATA filehandle for standalone Bytecode program (easy)
DATA filehandle for multiple bytecode-compiled modules (harder)
DATA filehandle for C-compiled program (yet harder)

* Features

type checking: Devel::TypeCheck
compile time v. runtime initialisation
save PMOPs in compiled form
selection of what to dump
options for cutting out line info etc.
comment output
shared constants
module dependencies
new Asm, JIT or PIR backend, PLJC ByteLoader magic
check Bytecode op coverage in the tests, should cover all ops 0-142
B::IO::SUBPROCESS

* Optimisations

collapse LISTOPs to UNOPs or BASEOPs
compile-time qw(), constant subs
global analysis of variables, type hints etc.
  Devel::TypeCheck instead of magic variable _ir, ... suffices
  e.g. mark IVonly SV's and magic/flags change functions and 
  optimize all op subtrees for the compiled backends to use direct ints
demand-loaded bytecode (leader of each basic block replaced by an op
  which loads in bytecode for its block)
fast sub calls for CC backend
