API to calculate size of fixed-size types

[danvk]

While creating custom types, it's sometimes helpful to know the size (in bytes) of a base type. In general this isn't possible to compute (because of dynamically-sized elements), but for simple types like int32 and objects composed of these simple types, it is.

It would be helpful if there were a computeSize(type) method on the jBinary object, or perhaps a computeSize method on type descriptors which would produce this information when it's available.

[RReverser]

Yeah, I started smth similar earlier but thought that not much use cases actually need it (while the size of the library might blow).