Ignore empty call-seq

In string.c two methods are implemented by the same function, but the C
parser thinks they have an empty call-seq.  When this is set on the
method it caused the Darkfish formatter to omit the method header.

Fixes #244
Fix deduplication of C methods with ==

C methods can share the same C function and comment.  When deduplicating
the call-seq methods that did not start with a letter were incorrectly
deduplicated due to special handling for [] and []=.  Additionally, the
selection of operator-style methods was insufficient.

Now special handling for [] and []= is only performed for those methods.

Now an extra check is made for operator-style methods.

Fixes #244