harden typing / coercion for on conflict/on duplicate key

harden typing / coercion for on conflict/on duplicate key

in 2.1 we want these structures to be cacheable, so start
by cleaning up types and adding coercions to enforce those types.
these will be more locked down in 2.1 as we will need to move
bound parameter coercion outside of compilation, but here
do some small starts and introduce in 2.0.

in one interest of cachability, a "literal_binds" that found
its way into SQLite's compiler is replaced with "literal_execute",
the difference being that the latter is cacheable.   This literal
is apparently necessary to suit SQLite's query planner for
the "index criteria" portion of the on conflict clause that otherwise
can't work with a real bound parameter.

Change-Id: I4d66ec1473321616a1707da324a7dfe7a61ec94e