Add arbitrary string type to the native object API (#4218)

* Custom strings are very common for optimizations around small objects
  or growth style optimizations, i.e.: grow at 1.57 times vs doubling vs..

  A second common strategy is to cooperate w/ the memory allocator
  see FBString[1] and seastar[2] string for examples.

[1] fbstring: https://github.com/facebook/folly/blob/master/folly/docs/FBString.md
[2] sstring: https://github.com/scylladb/seastar/blob/master/core/sstring.hh