Slow writing segments

[wasm-forge]

The stable-fs v0.6.4 is still slow when writing multiple small segments into a new file.
The metadata has to be updated for every little write, which has to find the right node every time and consumes a lot of cycles, here caching into a canister memory wouldn't work, as it wouldn't be prepared for canister upgrade that can happen at any moment.
One solution would be to write the temporary information into a dedicated memory, a similar solution that currently works for mounted memory files.

[wasm-forge]

This issue will be addressed in #20.