pgbench - add \if support

This patch adds \if support to pgbench, similar to psql's version added
in March.

This patch brings a consistent set of features especially when combined
with two other patches already in the (slow) CF process:

- https://commitfest.postgresql.org/10/596/ .. /15/985/
adds support for booleans expressions (comparisons, logical
operators, ...). This enhanced expression engine would be useful
to allow client-side expression in psql.

- https://commitfest.postgresql.org/10/669/ .. /15/669/
adds support for \gset, so that pgbench can interact with a database
and extract something into a variable, instead of discarding it.

This patch adds a \if construct so that an expression on variables,
possibly with data coming from the database, can change the behavior of a
script.

For instance, the following script, which uses features from the three
patches, would implement TPC-B per spec (not "tpcb-like", but really as
specified).

\set tbid random(1, :scale)
\set tid 10 * (:tbid - 1) + random(1, 10)
-- client in same branch as teller at 85%
\if :scale = 1 OR random(0, 99) < 85
-- same branch
\set bid :tbid
\else
-- different branch
\set bid 1 + (:tbid + random(1, :scale - 1)) % :scale
\endif
\set aid :bid * 100000 + random(1, 100000)
\set delta random(-999999, 999999)
BEGIN;
UPDATE pgbench_accounts
SET abalance = abalance + :delta WHERE aid = :aid
RETURNING abalance AS balance \gset
UPDATE pgbench_tellers
SET tbalance = tbalance + :delta WHERE tid = :tid;
UPDATE pgbench_branches
SET bbalance = bbalance + :delta WHERE bid = :bid;
INSERT INTO pgbench_history (tid, bid, aid, delta, mtime)
VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
END;

The patch moves the conditional stack infrastructure from psql to
fe_utils, so that it is available to both psql & pgbench.

A partial evaluation is performed to detect structural errors (eg missing
endif, else after else...) when the script is parsed, so that such errors
cannot occur when a script is running.

A new automaton state is added to quickly step over false branches.

TAP tests ensure reasonable coverage of the feature.

--
Fabien