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_structs.h

@@ -0,0 +1,6 @@
+#ifndef JEMALLOC_INTERNAL_EXTENT_DSS_STRUCTS_H
+#define JEMALLOC_INTERNAL_EXTENT_DSS_STRUCTS_H
+
+extern const char *dss_prec_names[];
+
+#endif /* JEMALLOC_INTERNAL_EXTENT_DSS_STRUCTS_H */