Fix parsing and pretty-printing of string literals

[Gabriella439]

While debugging the test failure on #218 I realized that the logic for parsing and pretty-printing import paths was broken in several ways:

• Import paths were being rendered unquoted (so the rendered .proto was not valid)

• Import parsing and pretty-printing was not handling escape sequences correctly

• The quickcheck generator was not even exercising this code path

  … which is why they remained broken

… so I fixed this

Fixes #218

[Gabriella439]

I had to remove a few tests which were broken. The root cause was that the proto3 parser was silently ignoring the rest of the file when hitting an invalid parse instead of failing. This PR fixed that (by requiring the parser to match eof when it's done), but then unearthed some invalid proto3 files in the test suite, which I had to delete (along with their reverse dependencies).

[j6carey]

Thanks!

[Gabriella439]

You're welcome!