#1968: locator-private bindings

[pshirshov]

This P/R introduces the concept of Locator-private bindings.
Locator-local instances cannot be seen by inherited locators.
There are 3 supported modes:

1. All the bindings are public unless explicitly marked as confined (the old behavior, kept as the default one)
2. All the bindings are private unless explicitly marked as exposed
3. Only GC roots are public

For bootstrap injectors the default behavior is (3). Also, from now on the bootstrap planning uses predefined set of roots instead of Roots.Everything, so bootstrap context are subjected to garbage colleciton.

THIS CHANGE IS INCOMPATIBLE WITH PREVIOUS VERSIONS and requires manual intervention if you have any custom bootstrap modules in your code.

All the bootstrap entities which you need to be available in the downstream locators, must be explicitly marked as exposed. There is no way to define GC roots for the bootstrap injector, so it considers all the exposed entities in bootstrap modules to be roots.

[codecov]

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