fix: adjust name mangling in kernel's nested inductive type processing

[kmill]

This PR fixes an issue where the kernel would not accept nested inductive types where the type being nested through was public with a private constructor. The kernel assumed that the name of the inductive type was a prefix of each constructor. Now the kernel uses a simpler procedure where the auxiliary type's name is prepended to constructor names, rather than replacing a prefix. Note that a consequence to this is that auxiliary recursors have non-atomic minor premise names (which was already the case for private constructors).

With @berberman

Closes #10789

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