* Cleanup, remove lib/system/allocators.nim. seqs_v2 and strs_v2 now use
allocShared0 by default.
* Fixed -d:useMalloc allocShared / reallocShared / deallocShared. These now use the alloc/dealloc/realloc implementation that also takes care of zeroing memory at realloc.
* Removed debug printfs
* Removed unpairedEnvAllocs() from tests/destructor/tnewruntime_misc
* More mmdisp cleanups. The shared allocators do not need to zero memory or throw since the regular ones already do that
* Introduced realloc0 and reallocShared0, these procs are now used by
strs_v2 and seqs_v2. This also allowed the -d:useMalloc allocator to
drop the extra header with allocation length.
* Moved strs_v2/seqs_v2 'allocated' flag into 'cap' field
* Added 'getAllocStats()' to get low level alloc/dealloc counters. Enable with -d:allocStats
* *allocShared implementations for boehm and go allocators now depend on the proper *allocImpl procs
