[#156] Export EntryPoints-related constraints

Description

Problem: when one wants to define ParameterHasEntryPoints for a polymorphic type (e. g. A a) they need to make sure that the constraint required for EntryPointsDerivation Epd* (A a) holds. But the required constraint (PlainEntryPointsC) is not exported, so it is hard to define such instances.

Solution: export PlainEntryPointsC. Additionally export EPTree and BuildEPTree which also might be useful in some cases.

Related issue(s)

Resolves #156 (closed)

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