[eigen] removing meta-unrollers

[Benoît Jacob <jacob@xxxxxxxxxxxxxxx>]

Hi List,

I'm about to remove the meta-unrollers and replace them by manual unrolling.

Rationale:
- meta-unrollers give very very long and useless compiler output when there is 
an error in the calling program, e.g. a matrix with 0 rows.
- in most cases (that is, everywhere except in CopyHelper) manually unrolling 
takes LESS lines of code (and is much more explicit) than meta-unrolling.
- in CopyHelper, we weren't handling all cases anyway. If e.g. the number of 
rows is known at compile-time but not the number of columns, our 
meta-unroller did not partially unroll (and I know Cyrille has a use case for 
that). Modifying it to handle that case would probably again make it longer 
than manual-unrolling.
- manual unrolling is easier on the compiler, and the code is far easier to 
read and maintain.

Cheers,

Benoit

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