Add Enumerator versions to Result#each and #each_row.
Use frozen String objects as field names and reuse them where possible.

This speeds Result#each up by 30%.
Populate the row-Array in Result#each_row and Result.values per rb_ary_new_from_values()
from VALUE array on the stack. The number of fields is limited on the server side
to ~1700, so that stack space is not critical. This gives a small speed up by 5%.