SharedTree: ChangeEncoder.decodeChange() should use 'Jsonable<T>'

[DLehenbauer]

The 'Jsonable' is another helper type that can catch some attempts to JSON serialize non-JSON serializable types.

It's typically used with type inference as follows:

function foo<T>(bar: Jsonable<T>) { ... }

foo(() => "not jsonable");    // Produces a compile time error.

See 'packages/runtime/datastore-definitions/src/jsonable.ts' for details.

[DLehenbauer]

I'm curious about naming asymmetry between 'encodeForJson' and 'decodeJson'. Was that intentional?

[alex-pardes]

@CraigMacomber actually wrote this file and picked these names, but the names seem fairly clear to me. I wouldn't object to something like encodeAsJsonable and decodeFromJsonable.

[DLehenbauer]

@alex-pardes - Motivation for PR is to pass on knowledge of Jsonable, which isn't easily discoverable.

[alex-pardes]

I think this PR would be much more valuable if we can delete JsonCompatible by doing something like this:

Suggested change

	export abstract class ChangeEncoder<TChange> {
	public abstract encodeForJson(formatVersion: number, change: TChange): JsonCompatible;
	export abstract class ChangeEncoder<TChange, Jsonable<TEncoded>> {
	public abstract encodeForJson(formatVersion: number, change: TChange): TEncoded;

[alex-pardes]

@CraigMacomber actually wrote this file and picked these names, but the names seem fairly clear to me. I wouldn't object to something like encodeAsJsonable and decodeFromJsonable.