fusion/experimental

[timotheecour]

std/experimental was IMO a good idea (but was only used for lib/experimental/diff.nim) and allows 0-change migration for clients using the usual import/export or include trick in case it wants to migrate to std/diff once stabilized.

we should IMO do the same for fusion:
modules can mature in fusion/experimental, breaking changes are allowed there, and client code that decides to use those (directly or via transitive closure) should not complain about those.

links

nim-lang#16004 (comment)

[timotheecour]

#575 may be a better idea, avoid the problem raised

But now we created a situation where it's better to import experimental / module! Because that's the import that actually works with older versions of Nim! So client code becomes: