htools: Add if' and select as functions

if' is just a functional version of the if-then-else construct. Using it
we can manipulate ifs as functions, and also build other functionality.

select allows to choose a result depending on truth value of statements,
with a final default, and can be used to substitute cascaded-nested
if-then-else statements.

Both come with unit tests.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>