That'll allow us to go from (thin_symbol, sym_table) to symbol, and vice
versa, where with the old representation there was no other way to go
from symbol to thin than redoing a full-lookup.
That'll help us transition to thin-symbol storage everywhere.
Since the analysis context initialization may trigger events (e.g.
"prelude file parsed"), language bindings (in particular Python) may
need to create/register their wrapper around the context that is just
created so that they can use that wrapper when invoking the callback.
Having allocation and initialization as a single atomic step makes this
impossible: this change splits them in the C API and update the
Python/OCaml bindings to allow this.
TN: VA12-016
