Fix API break & expand elsewhere

[oconnor0]

This does 4 different things. Any of which can be rejected without affecting the others (except that 3 depends on 2):

1. Change Xledger.Collections.Test tests via public API & reflection rather than having internal access. This changes how resolution of internal vs public is handled to more closely match third-party users of the library.
2. Support ReadOnlySpan on .NET 48. - which is why the .NET 48 version of this package depends on System.Memory.
3. Support collection expressions on .NET 48. - by implementing the single attribute polyfill that the compiler needs to specify the correct builder method.
4. Test new ImmArray construction. - since there is a breaking change in the API between 0.8.0 and 0.9.0 in C# 12 (tho not in C# 14).
5. Remove new ImmArray(params ReadOnlySpan) to revert API break.