Put std use behind a feature

[leighmcculloch]

@graydon pointed out that there value in us making the xdr lib usable inside contracts, at least so we can use enum values, but maybe someone will need other things too.

The main thing we need to exclude is any std dependency. We should add a feature std and when it is disabled drop any functionality that requires std.

[leighmcculloch]

Note that this issue does not preclude the use of alloc which is separately addressed by #39.