Update vtable restrictions once CBMC side lands.

[avanhatt]

Once this lands: diffblue/cbmc#6376

• Update minimum CBMC version.
• Enable fixme unit tests under src/test/rmc/DynTrait/*_fixme.rs.
• Add unit test for InternedString deserialization.
• Move flag check inside block, as listed here: Add option to restrict vtable function pointers #536

[tautschnig]

diffblue/cbmc#6376 has finally been merged, and will be part of the next CBMC release (5.46.0, due to be released 2021-12-09).

[tautschnig]

CBMC 5.46.0 has now been released.