Replace Expronicon with Moshi

[visr]

This is used for the exported Algebraic Data Type (ADT) TimeDomain.

Closes #741 which wasn't merged due to invalidations caused by newer Expronicon releases. Moshi is the replacement by the same author that fixes this issue. It is a different design so some care is taken to minimize disruption.

One benefit of this change is that it removes these dependencies from SciMLBase: Pkg, Expronicon, MLStyle, ArgTools, Downloads, FileWatching, InteractiveUtils, LibCURL, REPL, Tar, LibCURL_jll, MozillaCACerts_jll, Zlib_jll, nghttp2_jll, p7zip_jll

Moshi only depends on ExproniconLite and Jieko.

One of the differences between Expronicon and Moshi is that the former ADT macro creates a type, whereas the latter creates a module with the Type in there. Since the ADT was manually placed in the Clocks module, I could just rename the ADT to the module Clocks, and the old ADT name is kept as const TimeDomain = Clocks.Type. With that TimeDomain can still be used for dispatch or container typing like is done in ModelingToolkit.

The DiscriminatorType code was only added for Clock serialization in #753, but that now works without extra code.

The biggest change that I couldn't work around is that Continuous and SolverStepClock, both singleton variants, should now be explicitly constructed with Continuous instead of Continuous(). My idea was to land and release #921 first, so that ModelingToolkit can update to this new syntax, and then merge this next. Though if you prefer to just update both SciMLBase and ModelingToolkit together then #921 wouldn't be needed.

EDIT: This is a draft because this will break MTK unless SciML/ModelingToolkit.jl#3353 is merged and released first.

[ChrisRackauckas]

My idea was to land and release #921 first, so that ModelingToolkit can update to this new syntax, and then merge this next. Though if you prefer to just update both SciMLBase and ModelingToolkit together then #921 wouldn't be needed.

It's not just MTK but also SymbolicUtils.jl that needs it, and it's been a high priority thing to get that update in some time.

But yes let's do this in steps.