Implement the *allocx() API.

Implement the *allocx() API, which is a successor to the *allocm() API.
The *allocx() functions are slightly simpler to use because they have
fewer parameters, they directly return the results of primary interest,
and mallocx()/rallocx() avoid the strict aliasing pitfall that
allocm()/rallocx() share with posix_memalign().  The following code
violates strict aliasing rules:

    foo_t *foo;
    allocm((void **)&foo, NULL, 42, 0);

whereas the following is safe:

    foo_t *foo;
    void *p;
    allocm(&p, NULL, 42, 0);
    foo = (foo_t *)p;

mallocx() does not have this problem:

    foo_t *foo = (foo_t *)mallocx(42, 0);

include/jemalloc/internal/private_symbols.txt

@@ -207,17 +207,17 @@ huge_ralloc_no_move
 huge_salloc
 iallocm
 icalloc
-icallocx
+icalloct
 idalloc
-idallocx
+idalloct
 imalloc
-imallocx
+imalloct
 ipalloc
-ipallocx
+ipalloct
 iqalloc
-iqallocx
+iqalloct
 iralloc
-irallocx
+iralloct
 isalloc
 isthreaded
 ivsalloc