Drop the keyword file in multidimensional register definitions

[AndreasKrall]

Currently we have two options to define register files, one using the combined keyword register file and one just using the keyword register alone:

register file X : Bits<4> -> Bits<32>
register X : Bits<16><32>

Both are register files but are called differently. Therefore, it does not make sense to keep the keyword file (examples with dropped file):

register X : Bits<4> -> Bits<32>
register X : Bits<16><32>

register Y : Bits<2> * Bits <3> -> Bits<16>
register Y : Bits<4><8><16>

Proposal: Delete the keyword file, but keep both ways of defining multi dimensional registers as they have different specification capabilities (the `->´ version only allows the number of registers to be a power of 2) and different look and feel. This also would give a tiny simplification in the frontend.

[AndreasKrall]

In the meeting today we decided to drop the keyword file, but keep the alternate arrow notation together with the vector notation.