core: Add buffers to IP and ICMP

[rowanG077]

This significantly improves timing on ECP5. This allows ECP5 to once again reach 50Mhz with ease.

It does burn a few FFs and LUTs though On a UDP echo design on a 25k ECP5 we go from

Info:             TRELLIS_FF:  2334/24288     9%
Info:           TRELLIS_COMB:  5432/24288    22%
Info:           TRELLIS_RAMW:   136/ 3036     4%

to

Info:             TRELLIS_FF:  2570/24288    10%
Info:           TRELLIS_COMB:  5752/24288    23%
Info:           TRELLIS_RAMW:   136/ 3036     4%

So if you want to make this configurable @enjoy-digital let me know