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

lib/system/widestrs.nim

@@ -18,8 +18,6 @@ type
 
 when defined(nimv2):
 
-  import system / allocators
-
   type
     WideCString* = ptr UncheckedArray[Utf16Char]
 
@@ -29,8 +27,7 @@ when defined(nimv2):
 
   proc `=destroy`(a: var WideCStringObj) =
     if a.data != nil:
-      let alor = getLocalAllocator()
-      alor.dealloc(alor, a.data, a.bytes)
+      deallocShared(a.data)
       a.data = nil
 
   proc `=`(a: var WideCStringObj; b: WideCStringObj) {.error.}
@@ -41,8 +38,7 @@ when defined(nimv2):
 
   proc createWide(a: var WideCStringObj; bytes: int) =
     a.bytes = bytes
-    let alor = getLocalAllocator()
-    a.data = cast[typeof(a.data)](alor.alloc(alor, bytes))
+    a.data = cast[typeof(a.data)](allocShared0(bytes))
 
   template `[]`(a: WideCStringObj; idx: int): Utf16Char = a.data[idx]
   template `[]=`(a: WideCStringObj; idx: int; val: Utf16Char) = a.data[idx] = val