Add inline bucket support.

This commit adds support for writing small buckets directly inline to their value in
their parent's leaf node. Previously, subbuckets would simply have a bucket header
stored in their parent bucket which pointed to the root page. This required that
every bucket use at least a single page. This has a high overhead for buckets with
only one or two small items.

Inline buckets checks subbuckets to see if they only have a small amount of data
(about 1kb) and no subbuckets. If these conditions are met then the bucket's root
node is written to a fake page which is simply a pointer to the end of the bucket's
header.

Fixes #124.