Removed lib/system/allocators.nim. seqs_v2 and strs_v2 now uses allocShared0. (#13190)

* 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

tests/destructor/tv2_raise.nim

@@ -1,12 +1,12 @@
 discard """
-  cmd: '''nim c --newruntime $file'''
+  valgrind: true
+  cmd: '''nim c -d:allocStats --newruntime $file'''
   output: '''OK 3
-5 2'''
+(allocCount: 8, deallocCount: 3)'''
 """
 
 import strutils, math
 import system / ansi_c
-import system / allocators
 
 proc mainA =
   try:
@@ -50,6 +50,4 @@ except:
   inc ok
 
 echo "OK ", ok
-
-let (a, d) = allocCounters()
-discard cprintf("%ld %ld\n", a, d)
+echo getAllocStats()