Add the stall controller

[xThaid]

It made a few changes that will be useful in the future work on the frontend:

• I decoupled the stall logic from the fetch unit
• Func units call the resume method instead of defining it

todo:

• comments
• fix the test

One test in test_core is failing because of kuznia-rdzeni/transactron#45

[github-actions]

Benchmarks summary

Performance benchmarks

aha-mont64	crc32	minver	nettle-sha256	nsichneu	slre	statemate	ud
0.417 (0.000)	0.532 (0.000)	▲ 0.373 (+0.000)	▲ 0.632 (+0.000)	▲ 0.359 (+0.000)	▲ 0.291 (+0.000)	0.328 (0.000)	0.443 (0.000)

You can view all the metrics here.

Synthesis benchmarks (basic)

Device utilisation: (ECP5)	LUTs used as DFF: (ECP5)	LUTs used as carry: (ECP5)	LUTs used as ram: (ECP5)	Max clock frequency (Fmax)
▼ 13950 (-453)	▼ 4293 (-38)	1424 (0)	▼ 1104 (-32)	▲ 51 (+2)

Synthesis benchmarks (full)

Device utilisation: (ECP5)	LUTs used as DFF: (ECP5)	LUTs used as carry: (ECP5)	LUTs used as ram: (ECP5)	Max clock frequency (Fmax)
▼ 24805 (-1187)	▼ 6783 (-38)	1940 (0)	▼ 1632 (-32)	▼ 40 (-3)