expr: Replace expression engine

Rework the expression engine to use recursive descent evaluation rather than
a shunting yard algorithm. Among other things, it is easier to make lazy operators
and the ternary operator work correctly.

In particular, the following expression no longer crashes: $(99?9,99?9:*9:999)?9)

And the code is now smaller.

Signed-off-by: Steve Bennett <steveb@workware.net.au>