callback - Moved internal dispatch mechanism to generated op-table

This allows additional attributes to be attached to the internally
generated type such as move and destructor operations with no increase
in RAM footprint.

The current overloads can't take advantage of this, but it does open
the possibility for more powerful overloads that can provide these
additional attributes.

Changes to mbed-os memory consumption:

        .text   .data   .bss
before  57887   2292    7692
after   57842   2292    7691

Author: geky