Implement attribute operations using the repr type

[emillon]

There was a repr type in CK_ATTRIBUTE used for internal operations. It turns out that it's pretty useful in P11_attribute as well.

The idea is to do all operations in two steps:

• get the repr associated to a given attribute type (is it represented using an unsigned long? an ASCII string? a string containing hex data?)
• do the actual thing based on the repr.

This is a big simplification because pattern matching is only as wide as the repr type, not as P11_attribute_type.t.

[emillon]

Thanks for the :eye: !