Don't detach Strings from Pool<String>

[zslayton]

This was done to minimize refactoring required to release v0.10.0, which included auto-reconnect, reduced memory allocation and became usable from rustc's stable channel. However, the resulting code is a bit cluttered.

Instances of String in the Frame struct should be replaced with lifeguard's Recycled<String> type. This will allow all of the strings.attach(...) calls to be deleted.