Remove default implementation of `Expr::compareContents(const Expr&)`

and make it a pure virtual method. Also make it protected rather than
public because it is an implementation detail of `Expr::compare()`.

This means that sub classes must implement it so this commit also
provides implementations. The comparision behaves as before except
for `ConcatExpr` where its `width` attribute is now used (previously
it wasn't).

This commit also documents the semantics of `Expr::compareContents(const Expr&)`
which was clearly needed because it has been incorrectly implemented
in the past and has gone unnoticed for several years.

This partially addresses #515 .

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