expose functions from string→double?

IIUC, this only exposes functions from doubles to strings, not the other way around, but https://github.com/google/double-conversion/blob/master/double-conversion/string-to-double.h has functions from string-to-double – would it make sense to include both directions?

The default `read` is pretty slow on doubles; there is [bytestring-lexing](https://hackage.haskell.org/package/bytestring-lexing) which is faster than default, but I'm guessing double-conversion could be even faster – or perhaps [fast_float](https://github.com/fastfloat/fast_float) would be a better option?