Bigstring support for binary converters

[tomjridge]

Jane Street bin_prot uses Bigstring as the target buffer type. This is useful for working with C libraries, mmaps etc. For binary converters, we can avoid intermediate buffers if we have an interface that takes the position in the Bigstring to read/write from (rather than converting to string each time).

In short, this is a request for Bigstring support with a different interface than currently is used for encode_bin and decode_bin.

[tomjridge]

For even better performance, I think read takes a "int ref" as the position to read from. This avoids allocating a pair for the return type (the next position to read is updated via the int ref). Hope this makes some kind of sense.

[craigfe]

This is a good idea & should be pretty straightforward thanks to the recent refactoring to extract binary_codec.ml.

We should be sure that whatever solution we have using refs is multicore-compatible. Other than that it seems best to allocate as rarely as possible.