compose: drop the 65535 node limit (#343)

compose: drop the 65535 node limit (#343)

In commit 1638409b22aef33d487863876ab214b949db4984, the number of
compose nodes was limited to 65535 to enable "future optimizations",
which apparently means slightly reduced memory usage due to fitting in
a uint16_t. At this time, it was mentioned that the author was not
aware of "any compose files which come close".

However, I'm one of the users that actually do require a larger number
of nodes for their compose file. Thus, use a uint32_t again and raise
the limit significantly.