Move the profiles lookup tables into a new Dictionary message

[dmathieu]

As discussed in #648

[christos68k]

We should also update the references in documentation strings from ProfilesData.XY_table to refer to ProfilesDictionary.XY_table.

[jhalliday]

Thanks, Damien. I like it, this feels like a more refined direction than #648 was.

I agree with points on streamlining terminology. Mixing 'reference' and 'lookup' isn't great. 'lookup_tables' field should probably become 'reference_tables' for consistency with the message name? Likewise if going with 'Dictionary'. I think the use of _index/_indices for pointers into the tables is already consistent, though if we're going with 'reference' everywhere I'm inclined to _ref/_reference/_references suffixes as an alternative and that may be a point in favour of using 'Reference' over 'Dictionary' for the message type. Whilst I'm the reason _strindex exists as a special distinction, I'm no longer sold on its advantages, it could be simplified to _index/_ref without great loss.

string_table is the only field likely to be common across ${signal}ReferenceTables messages in future, there may be some small clarity/consistency advantage to establishing a convention of ordering it as the first field in the message.

The idea of adding a 'bytes tables_id' field in ProfilesReferenceTables to allow for separately transmitted messages referencing into a particular set of tables rather than one default/bundled one still has some attraction, but at present I'm leaning towards leaving it out as YAGNI.

[felixge]

LGTM. I like this direction! Just got some nits.

[PaulinaParangerHr]

Really