Do correct bounds checking in `natural` in Capability.hs

Capability.hs defines natural like this:

natural :: Num n => Parser n
natural = decimal

However, decimal is documented as:

Warning: this function does not perform range checks.

This means that natural :: Parser Word8 will accept "256" as input and produce 0 as output. Instead, this should be a fail of some sort so that when natural is used to parse a type of known size it either succeeds for inputs in the range of that type or rejects them.