Top level #to_s returns "main"

by setting `TOPLEVEL_BINDING` const right before frame execution,
which is probably the wrong place...
Moving definition of TOPLEVEL_BINDING to lib-ruby
Changes per feedback
Untagging a few more specs for Kernel.eval
Correct error raising per feedback
More fixes per feedback

making Alex write my code through comments!  Time for bed.
Merge pull request #529 from modcloth/mbh/main-to_s-with-toplevel-binding

Kernel#eval, TOPLEVEL_BINDING, and top #to_s returns "main"