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_dss_externs.h

@@ -0,0 +1,14 @@
+#ifndef JEMALLOC_INTERNAL_EXTENT_DSS_EXTERNS_H
+#define JEMALLOC_INTERNAL_EXTENT_DSS_EXTERNS_H
+
+extern const char	*opt_dss;
+
+dss_prec_t	extent_dss_prec_get(void);
+bool	extent_dss_prec_set(dss_prec_t dss_prec);
+void	*extent_alloc_dss(tsdn_t *tsdn, arena_t *arena, void *new_addr,
+    size_t size, size_t alignment, bool *zero, bool *commit);
+bool	extent_in_dss(void *addr);
+bool	extent_dss_mergeable(void *addr_a, void *addr_b);
+void	extent_dss_boot(void);
+
+#endif /* JEMALLOC_INTERNAL_EXTENT_DSS_EXTERNS_H */