One of the benefits of prepared statements is that the SQL string is
parsed just once and re-used. This updates SQLStatement to do just that
and store the parsed result.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Rename sql_statement to prepare_statement and statement_execute to
execute_statement. The former aligns more with other database libraries
(e.g. Java's JDBC prepareStatement). The latter reads less awkwardly.
