We are going to split sql_prepare_and_execute() into several explicit
and logically separated steps:

1. sql_prepare() -- compile VDBE byte-code
2. sql_bind() -- bind variables (if there are any)
3. sql_execute() -- query (byte-code) execution in virtual machine

For instance, for dry-run we are interested only in query preparation.
Contrary, if we had prepared statement cache, we could skip query
preparation and handle only bind and execute steps.

To avoid inclusion of sql/sqlInt.h header (which gathers almost all SQL
specific functions and constants) let's move sql_prepare() to
box/execute.h header (which already holds sql_prepare_and_execute()).

Needed for #3292