Make object path resolution more robust (fix #957)

This fixes #957 and should make object path resolution more predictable
in general. Instead of splitting the path on "/" before doing any
parsing, we use `StringScanner` and `complete_expression?` to scan
through the string looking for complete slash-delimited Ruby
expressions.

It also turned out that separating the code for handling "-" from the
path-resolution code simplified things a lot. It doesn't really make
sense for "-" to be in there anyway, since paths like "foo/-/bar" don't
mean anything.
Merge pull request #1203 from pry/fix-957

Make object path resolution more robust (fix #957)