Use a builder object to inquire, validate, and register a custom toolchain

[kevherro]

Previously, inquiring and validating a custom toolchain was coupled to registration, which made it difficult to simply look at the custom toolchain independent of registering it.

This change implements ToolchainBuilder, a builder object that finds, validates, and registers a custom toolchain. Each step can be performed independently, with the caveat that registration depends on validation which depends on finding the custom toolchain.

Use cases include:

• Finding a custom toolchain,
• Finding and validating a custom toolchain,
• Finding, validating, and registering a custom toolchain

This change also cleans up some of the platform-specific symlink code by implementing a trait, SymlinkOrFallback, which different platforms can adopt.

This change does not add or remove any existing functionality (except for maybe introducing opportunities for non-Unix-like or Windows platforms to register a custom toolchain).

Discussion at #607

[kevherro]

@mitsuhiko any movement here?

[mitsuhiko]

Will try to get to it next week.