Allow for reading blobs without allocating

[udoprog]

This introduces the FixedBytes type which implements Readable. This allows for allocating the space necessary to read N bytes out of a column onto the stack using an uninitialized byte array.

This can be useful in many scenarios, I primarily use it to avoid allocations for identifiers and timestamps which are fixed size blobs on the client side.

Naming is just suggestions at this point.