Implement generalized btree push, note the boundary conditions when id=weight

This really just required care around calculating the expected B-tree id
and rbyd id (which are different!).

B-tree append, aka B-tree push with id=weight, is actually the outlier.
We need a B-tree id that can identify the rbyd we're appending to, but
this id itself doesn't exist in the tree yet, which can be a bit tricky.

lfs.h

@@ -367,12 +367,12 @@ typedef struct lfsr_branch {
 } lfsr_branch_t;
 
 typedef struct lfsr_btree {
+    lfs_size_t weight;
     // TODO do we need full tag actually? this fits in a byte?
     lfsr_tag_t tag;
     // how can we take advantage of byte packing with union alignment?
     union {
         struct {
-            lfs_size_t weight;
             uint8_t size;
             uint8_t buf[LFSR_BTREE_INLINE_SIZE];
         } inlined;