Optimize encoding of note_type_id such that it occupies 7 bits.

[benesjan]

With this PR note_type_id now fits into 7 bits but the encoding is still 32 bytes. I didn't optimize the encoding because the to_be_bytes function will go away once we serialize to fields.

Note: Started work on this in #10899 but stopped as the whole to_be_bytes func will be most likely eventually nuked.