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/base_inlines.h

@@ -0,0 +1,17 @@
+#ifndef JEMALLOC_INTERNAL_BASE_INLINES_H
+#define JEMALLOC_INTERNAL_BASE_INLINES_H
+
+#ifndef JEMALLOC_ENABLE_INLINE
+unsigned	base_ind_get(const base_t *base);
+#endif
+
+#if (defined(JEMALLOC_ENABLE_INLINE) || defined(JEMALLOC_BASE_C_))
+JEMALLOC_INLINE unsigned
+base_ind_get(const base_t *base)
+{
+
+	return (base->ind);
+}
+#endif
+
+#endif /* JEMALLOC_INTERNAL_BASE_INLINES_H */