breaking format changes in v2 about biscuit

[divarvel]

What should be shipped in current implementations ASAP:

• updated symbol table (remove authority, ambient and revocation_id, replace them with read, write and role
• [0..255] restricted range for symbol ids so that the default symbol table can grow without breaking tokens (not 100% sure it would make adding default symbols breakage-free, though) messing with the symbol table will create issues anyway, so this offset does not buy us anything
• making execution scope default to OnlyAuthority
• bump the max schema version

Configurable scoping rules can land afterwards, as it's purely backwards compatible

[divarvel]

Done in #97 and #103