Don't box block descriptors

[TellowKrinkle]

Switches block descriptors to const globals, so we don't have to heap allocate a new one for each block

Also deduplicates copy functions (since they're all the same) and completely removes the copy and dispose functions for blocks implementing Copy

Note: I had to change the descriptor function signatures to use *mut c_void/*const c_void because otherwise the compiler complains. An alternative is forcing all the block parameters to be 'static.

[madsmtm]

Implementation looks good to me.

For reference, I've done most of these improvements in my fork block2 as well. Good idea with the mem::needs_drop check, I'll probably incorporate that.