Issue #285: Add support for lengths other than 64 bits.

[tkaitchuck]

This change adds two new config values string_size and array_size.
Both of which take an enum to allow the lengths of strings and vectors/arrays to be set to:
U8, U16, U32, and U64. Where U64 is the default. (Which is compatible with current behavior.)

In the event that the data being serialized does not fit into the size specified the serialization will fail with ErrorKind::SizeTypeLimit

Fixes #285 .

[tkaitchuck]

@dtolnay Does this look like a good approach?

[dtolnay]

Hi @tkaitchuck, I am not really involved with this project these days and I don't get the impression that there are current maintainers who could dedicate time to reviewing something like this in the context of how it fits into a long term design for bincode.

That said, my first impression is that I would prefer if bincode did not support this. I think there is room for someone to make a different data format library that is more geared toward this sort of customization. I think it's fair for bincode to dictate a simple default format and we shouldn't feel the need to build in all manner of customization.

[tkaitchuck]

Moved here: pravega#4