Deprecating '#' as line comment character

[Xikui Wang]

Hi Devs,

With the patch in [1], we will no longer support using '#'  for line
comment. You may use '--' or '//' as alternatives. Meanwhile, backquoting
function name for invoking external UDF in SQLPP is not enforced now. You
can use either

testlib#getCapital("England") or `testlib#getCapital`("England")

to invoke your external UDF.

[1] https://asterix-gerrit.ics.uci.edu/#/c/2037/

Best,
Xikui