evaluate: remove anon sets with exactly one element

evaluate: remove anon sets with exactly one element

Auto-replace lookups in single-element anon sets with a standard compare.

'add rule foo bar meta iif { "lo" }' gets replaced with
'add rule foo bar meta iif "lo"'.

The former is a set lookup, the latter is a comparision.
Comparisions are faster for the one-element case.

Only prefixes, ranges and values are handled at this time.

Anonymous maps are left alone, same for concatenations.

Concatenations could be handled, but it would require more work:
the concatenation would have to be replaced with a singleton value.
Evaluation step rejects concat RHS on a relational expression.

Signed-off-by: Florian Westphal <fw@strlen.de>