Update to amaranth 0.4

[jfng]

• fix fallout from Signature immutability RFC rfcs#37 and Component signature immutability rfcs#38
• set requirement to "amaranth>=0.4<0.6"
• add CI tests against 0.4

[jfng]

• set requirement to "amaranth>=0.4<0.6"
• add CI tests against 0.4

Doing this won't help us, as upcoming work (such as #58) depends on upstream functionality that isn't part of the 0.4 release.