Address Cyclic Unit references in Parser.h

[InsertCreativityHere]

In the Slice compiler, the base type for all Slice types & definitions is SyntaxTreeBase. SyntaxTreeBase itself inherits from the true base class: GrammarBase. This represents other grammatical objects that aren't types or definitions (metadata, parts of definitions that haven't been fully parsed yet, and tokens for holding string-literals, integer-literals, etc.)

One of the types which inherits from SyntaxTreeBase is Unit (which represents a compilation unit, a.k.a. 1 Slice file).
But, this inheritance is useless and causes problems for us because SyntaxTreeBase holds a UnitPtr _unit;, a cyclic reference.

More importantly though is that this API is useless, and doesn't make sense for Unit to implement:

virtual void destroy();                   // Only sets `_unit = nullptr;`
UnitPtr unit() const;                     // Useless on `Unit`, it already has access to itself.
DefinitionContextPtr definitionContext() const;     // Always `nullptr` for `Unit`
virtual void visit(ParserVisitor* visitor);         // Only useful function.

We should consider either moving these useless functions down the inheritance tree, or move Unit up the tree, so that it isn't implementing these useless functions.

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If that isn't possible without great effort, we should at least fix the cyclic reference issue as suggested by @pepone:
#3292 (comment)