Primary Sendable support for Fluent

[gwynne]

These changes are now available in 1.48.0

Adds as much Sendable correctness to FluentKit as it is possible to add.

Important

It was not possible to make Model correctly Sendable due to limitations of how property wrappers work, even though the wrappers themselves are Sendable. Users can get around this by adding @unchecked Sendable to each individual Model class, but this of course does not confer actual safety; it just disables the compiler's warnings.

Also adds some missing APIs to FluentSQL to allow encoding Fluent models to SQLKit queries. The existing APIs for decoding Fluent models from SQLKit queries have been renamed, and the old names deprecated.

Other changes:

• The performance improvements for Mirror are now used in Swift 5.10
• QueryHistory is no longer thread-unsafe for reading
• The minimum supported Swift version is now 5.8
• SomeCodingKey is now an alias for SQLKit's version

[0xTim]

One change I want, the rest I can live with.

Not overly happy with the property wrappers requiring unchecked, especially given the lack of access management internally but we don't have a huge amount of leeway here.

Very happy to drop 5.9 though