ByteBufferAllocator: switch to size_t where appropriate (#503)
ByteBufferAllocator: switch to size_t where appropriate (#503)

Motivation:

In a few situations we used `Int` in places where malloc/realloc
actually use `size_t`. On 64-bit platforms this doesn't actually matter
but on 32-bit it does!

Modifications:

- replace `Int` with `size_t` where appropriate

Result:

- should be able to use more than 2GB on 32-bit platforms
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio