Test SRL's and SRL chains

[litghost]

Current status

7-series SRL primitives are untested, but should work in the current FPGA interchange implementation.

Work to be done

Test designs should be added to exercise SRL placement. It is expected that single SRL elements should work right away. Chained SRL primitives are more complicated. If the SRL chain length is less than 4, then the SRL's have exactly 1 legal placement, and probably need cell constraints (similiar to #262). However if the SRL chain length is longer than 4, then the chain has to be broken into chains of length 4 in some way. There are many legal valid chain arrangements, and using the cell constraint system will force the chain split into a particular configuration. This may be okay.