Add a register file, optimized for synthesis on FPGAs supporting
distributed RAM.

Principle:

The baseline implementation implements the register file as an array of
flip-flops and implements large multiplexers for read- and write-
accesses. On FPGAs, we have a more efficient implementation for data
storage: By using distributed RAM for memory storage, we can store up
to 64 bits in just one LUT (depending on the memory layout and FPGA
device). In addition, distributed RAM comes with integrated address
decoders. The register file features one distributed RAM block per
implemented sync write port, each with the parametrized number of
async read ports. The read access is arbitrated depending on which
block was last written to. For this purpose an additional array of
*NUM_WORDS* registers is maintained keeping track of write accesses.

Since both FFs and multiplexers are an expensive structure on FPGA
technology, the achieved savings are considerable. The register file
is used for the FPU and general purpose register files.

Concrete Savings: (Xilinx Kintex-7, xc7k325tffg900-2)

```
            LUT    FF      LUTRAM
---------------------------------
baseline:   40499  22799   0
optimized:  36350  18806   440
---------------------------------
Diff        -4149  -3993   +440
            -10.2% -17.5%
```

Signed-off-by: ganoam <gnoam@live.com>