Implicitly set the rm-bit in generic grow tags

Our rbyds support changing the weight of a tag without knowing the
actual tag. This is useful for btrees, which always make weight changes
without knowing if the leading tag is a name or a branch (it depends on
the type of btree).

But to make this work, it needs the rm-bit to be set. This is because
internally the rm-bit indicates we don't want to write-out a tag. Which
we don't for grow tags, because, well, they're not real tags.

Previously this was done by putting LFSR_TAG_GROW(RM) everywhere a
generic grow as needed, but since this is so common we might as well
just set the rm-bit in LFSR_TAG_GROW.

Note that LFSR_TAG_GROW(tag) (the macro) does not set the rm-bit.

This makes the code a bit more readable at the risk of an unintuitive
relationship between LFSR_TAG_GROW and LFSR_TAG_GROW(tag).

tests/test_btree.toml

@@ -49,7 +49,7 @@ code = '''
         // the struct tag
         return lfsr_btree_commit(lfs, btree, LFSR_ATTRS(
                 LFSR_ATTR(bid, WIDE(TAG(tag)), 0, DATA(data)),
-                LFSR_ATTR(bid, GROW(RM), weight - weight_, NULL)));
+                LFSR_ATTR(bid, GROW, weight - weight_, NULL)));
     }
 
     static int lfsr_btree_pop(lfs_t *lfs, lfsr_btree_t *btree, lfs_size_t bid) {
@@ -84,7 +84,7 @@ code = '''
         }
 
         return lfsr_btree_commit(lfs, btree, LFSR_ATTRS(
-                LFSR_ATTR(bid, GROW(RM), +weight1-weight_, NULL),
+                LFSR_ATTR(bid, GROW, +weight1-weight_, NULL),
                 LFSR_ATTR(bid-(weight_-1)+weight1-1, TAG(tag1), 0, DATA(data1)),
                 (lfsr_data_size(&name) > 0
                     ? LFSR_ATTR(bid-(weight_-1)+weight1,