Break up headers into constituent parts

This is part of a broader change to make header files better represent the
dependencies between one another (see
https://github.com/jemalloc/jemalloc/issues/533). It breaks up component headers
into smaller parts that can be made to have a simpler dependency graph.

For the autogenerated headers (smoothstep.h and size_classes.h), no splitting
was necessary, so I didn't add support to emit multiple headers.

include/jemalloc/internal/extent_structs.h

@@ -0,0 +1,84 @@
+#ifndef JEMALLOC_INTERNAL_EXTENT_STRUCTS_H
+#define JEMALLOC_INTERNAL_EXTENT_STRUCTS_H
+
+/* Extent (span of pages).  Use accessor functions for e_* fields. */
+struct extent_s {
+	/* Arena from which this extent came, if any. */
+	arena_t			*e_arena;
+
+	/* Pointer to the extent that this structure is responsible for. */
+	void			*e_addr;
+
+	/* Extent size. */
+	size_t			e_size;
+
+	/*
+	 * Usable size, typically smaller than extent size due to large_pad or
+	 * promotion of sampled small regions.
+	 */
+	size_t			e_usize;
+
+	/*
+	 * Serial number (potentially non-unique).
+	 *
+	 * In principle serial numbers can wrap around on 32-bit systems if
+	 * JEMALLOC_MUNMAP is defined, but as long as comparison functions fall
+	 * back on address comparison for equal serial numbers, stable (if
+	 * imperfect) ordering is maintained.
+	 *
+	 * Serial numbers may not be unique even in the absence of wrap-around,
+	 * e.g. when splitting an extent and assigning the same serial number to
+	 * both resulting adjacent extents.
+	 */
+	size_t			e_sn;
+
+	/* True if extent is active (in use). */
+	bool			e_active;
+
+	/*
+	 * The zeroed flag is used by extent recycling code to track whether
+	 * memory is zero-filled.
+	 */
+	bool			e_zeroed;
+
+	/*
+	 * True if physical memory is committed to the extent, whether
+	 * explicitly or implicitly as on a system that overcommits and
+	 * satisfies physical memory needs on demand via soft page faults.
+	 */
+	bool			e_committed;
+
+	/*
+	 * The slab flag indicates whether the extent is used for a slab of
+	 * small regions.  This helps differentiate small size classes, and it
+	 * indicates whether interior pointers can be looked up via iealloc().
+	 */
+	bool			e_slab;
+
+	union {
+		/* Small region slab metadata. */
+		arena_slab_data_t	e_slab_data;
+
+		/* Profile counters, used for large objects. */
+		union {
+			void		*e_prof_tctx_pun;
+			prof_tctx_t	*e_prof_tctx;
+		};
+	};
+
+	/*
+	 * Linkage for arena's extents_dirty and arena_bin_t's slabs_full rings.
+	 */
+	qr(extent_t)		qr_link;
+
+	union {
+		/* Linkage for per size class sn/address-ordered heaps. */
+		phn(extent_t)		ph_link;
+
+		/* Linkage for arena's large and extent_cache lists. */
+		ql_elm(extent_t)	ql_link;
+	};
+};
+typedef ph(extent_t) extent_heap_t;
+
+#endif /* JEMALLOC_INTERNAL_EXTENT_STRUCTS_H */