Fix COUNT(DISTINCT col) without GROUP BY

Assign a value for the key used to enforce uniqueness for DISTINCT
columns in do_calc(). This ensures the logic to skip seen column values
works correctly, especially in the absence of GROUP BY.
Fix space parsing error with '>=' and '<='

Ensure that the regular expression to insert space around operators
doesn't match the '=' in '<=' or '>='.
apply reworked Perl::Tidy rules
Merge branch 'fix-distinct' of https://github.com/grantisu/SQL-Statement into grantisu-fix-distinct

Conflicts:
	lib/SQL/Parser.pm
	lib/SQL/Statement.pm
Tidy patches after merge