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Move `TreeFormatter` out of core and decouple from `TreeVisitor` by introducing an interface to represent something that can receive diagnostic information.

Tweaks to error messages generated by the instantiator.

Change `TreeFormatter` so that it understands how to present a few more types in diagnostics messages.

Some improvements to the 'found multiple transforms' error message. Also apply some consistent formatting to some of the other variant error messages, by reusing `TreeFormatter`.

Some tweaks to the 'no compatible toolchains available' error message and the display names of the tool chain implementations.

- Some tweaks to the formatting of the error message received when no tool chains are available to build a particular platform. - Changed DefaultToolChainRegistry so that it does not create a ToolChain instance if no defaults have been registered.