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/tnewruntime_misc.nim

@@ -1,17 +1,15 @@
 discard """
-  cmd: '''nim cpp --newruntime --threads:on $file'''
+  cmd: '''nim cpp -d:allocStats --newruntime --threads:on $file'''
   output: '''(field: "value")
 Indeed
 axc
 (v: 10)
-0  new: 0
 ...
 destroying GenericObj[T] GenericObj[system.int]
 test
-'''
+(allocCount: 17, deallocCount: 15)'''
 """
 
-import system / allocators
 import system / ansi_c
 
 import tables
@@ -24,6 +22,8 @@ type
 import os
 putEnv("HEAPTRASHING", "Indeed")
 
+let s1 = getAllocStats()
+
 proc main =
   var w = newTable[string, owned Node]()
   w["key"] = Node(field: "value")
@@ -87,9 +87,6 @@ proc testWrongAt() =
 
 testWrongAt()
 
-let (a, d) = allocCounters()
-discard cprintf("%ld  new: %ld\n", a - unpairedEnvAllocs() - d, allocs)
-
 #-------------------------------------------------
 type
   Table[A, B] = object
@@ -134,3 +131,4 @@ proc xx(xml: string): MyObject =
 
 
 discard xx("test")
+echo getAllocStats() - s1