Return an error in case the length is too big. Also take this
opportunity to have a single allocating function for the size and
overflow logic.

This reduces the size of the struct from 32 to 26 bytes, and leaves a
single padding byte at the end of the struct (which comes from the
zero-length array).

We already know the size due to the `memchr()` so use that information
instead of calling `strlen()` on it.

These are rather small allocations, so we end up spending a non-trivial
amount of time asking the OS for memory. Since these entries are tied to
the lifetime of their tree, we can give the tree a pool so we speed up
the allocations.

We've already looked at the filename with `memchr()` and then used
`strlen()` to allocate the entry. We already know how much we have to
advance to get to the object id, so add the filename length instead of
looking at each byte again.

Memleak fixes

checkout: only consider nsecs when built that way