RFC: adopt semver, like almost all nimble packages

[timotheecour]

right now fusion.nimble is stuck at 1.0 (I wished it didn't start out at 1.0, packages should always start at some 0.x.y).
This proposal is to start adopting semver; anything is better than the current approach, eg:

• breaking changes likely to break code (which should be minimized but sometimes are the lesser evil) bump minor
• api additions bump patch
• nim should start referring to point releases instead of arbitrary looking FusionStableCommit = "319aac4d43b04113831b529f8003e82f4af6a4a5"
• we can also distinguish odd vs even minor and patch as is done in nim, with rationale that fusion 1.2x.2y even is more stable than 1.(2x+1).(2y+1)

note that any change (including an API addition) can be considered breaking changes in nim (eg, causing a redefinition error when fusion defines bar which was otherwise in some 3rd party import), so practicality trumps 0 breaking changes.

links

• fusion/experimental timotheecour/Nim#393