This adds:
* PG::Result#field_name_type=
* PG::Result#field_name_type
* PG::Result#field_names_as

Symbol comparison and lookup is faster, since the string hash is generated at Symbol creation.
So retrieval of Symbol based field names is somewhat slower, but this is easily outperformed in frameworks that do a lot of lookups.

This raises ruby version requirement to 2.2, since it depends on GC'able symbols.

Fixes ged#278

…apped values

Truffleruby doesn't support rb_check_symbol_cstr() so far, but all symbols in Truffleruby are GC'd anyway.
So we can use classic ID2SYM to allocate the Symbol objects.

Discussed here: oracle/truffleruby#1814

It allows to set the field name type on a connection scope.

Added methods are:
    * PG::Connection#field_name_type=
    * PG::Connection#field_name_type

It was using strings before.