Improved seekable format ingestion speed for small frame size

Improved seekable format ingestion speed for small frame size

As reported by @P-E-Meunier in https://github.com/facebook/zstd/issues/2662#issuecomment-1443836186,
seekable format ingestion speed can be particularly slow
when selected `FRAME_SIZE` is very small,
especially in combination with the recent row_hash compression mode.
The specific scenario mentioned was `pijul`,
using frame sizes of 256 bytes and level 10.

This is improved in this PR,
by providing approximate parameter adaptation to the compression process.

Tested locally on a M1 laptop,
ingestion of `enwik8` using `pijul` parameters
went from 35sec. (before this PR) to 2.5sec (with this PR).
For the specific corner case of a file full of zeroes,
this is even more pronounced, going from 45sec. to 0.5sec.

These benefits are unrelated to (and come on top of) other improvement efforts currently being made by @yoniko for the row_hash compression method specifically.

The `seekable_compress` test program has been updated to allows setting compression level,
in order to produce these performance results.