foreach loop support

[mglb]

Adds support for foreach loop construct. It does so by converting the loop into one or more (nested) for loops.

Since array ranges are not available at the moment when the loop processing happens, functions like $left, $right, etc are used in generated for loop(s). Yosys does not support arrays with multiple ranges, so in order to support those the functions are resolved in plugin's simplification step.

Todo:

• more tests
• cleanup
• use/test foreach-related improvements made in Surelog (skipped indexes).