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);

test/integration/rallocx.c

@@ -0,0 +1,51 @@
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_grow_and_shrink)
+{
+	void *p, *q;
+	size_t tsz;
+#define	NCYCLES 3
+	unsigned i, j;
+#define	NSZS 2500
+	size_t szs[NSZS];
+#define	MAXSZ ZU(12 * 1024 * 1024)
+
+	p = mallocx(1, 0);
+	assert_ptr_not_null(p, "Unexpected mallocx() error");
+	szs[0] = sallocx(p, 0);
+
+	for (i = 0; i < NCYCLES; i++) {
+		for (j = 1; j < NSZS && szs[j-1] < MAXSZ; j++) {
+			q = rallocx(p, szs[j-1]+1, 0);
+			assert_ptr_not_null(q,
+			    "Unexpected rallocx() error for size=%zu-->%zu",
+			    szs[j-1], szs[j-1]+1);
+			szs[j] = sallocx(q, 0);
+			assert_zu_ne(szs[j], szs[j-1]+1,
+			    "Expected size to at least: %zu", szs[j-1]+1);
+			p = q;
+		}
+
+		for (j--; j > 0; j--) {
+			q = rallocx(p, szs[j-1], 0);
+			assert_ptr_not_null(q,
+			    "Unexpected rallocx() error for size=%zu-->%zu",
+			    szs[j], szs[j-1]);
+			tsz = sallocx(q, 0);
+			assert_zu_eq(tsz, szs[j-1],
+			    "Expected size=%zu, got size=%zu", szs[j-1], tsz);
+			p = q;
+		}
+	}
+
+	dallocx(p, 0);
+}
+TEST_END
+
+int
+main(void)
+{
+
+	return (test(
+	    test_grow_and_shrink));
+}