Introduce class operation

This patch introduces class operation, the new base class for all
expression operations.

In the new approach, an operation is simply a class that presents a
certain interface.  Operations own their operands, and management is
done via unique_ptr.

The operation interface is largely ad hoc, based on the evolution of
expression handling in GDB.  Parts (for example,
evaluate_with_coercion) are probably redundant; however I took this
approach to try to avoid mixing different kinds of refactorings.

In some specific situations, rather than add a generic method across
the entire operation class hierarchy, I chose instead to use
dynamic_cast and specialized methods on certain concrete subclasses.
This will appear in some subsequent patches.

One goal of this work is to avoid the kinds of easy-to-make errors
that affected the old implementation.  To this end, some helper
subclasses are also added here.  These helpers automate the
implementation of the 'dump', 'uses_objfile', and 'constant_p'
methods.  Nearly every concrete operation that is subsequently added
will use these facilities.  (Note that the 'dump' implementation is
only outlined here, the body appears in the next patch.)

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* expression.h (expr::operation): New class.
	(expr::make_operation): New function.
	(expr::operation_up): New typedef.
	* expop.h: New file.
	* eval.c (operation::evaluate_for_cast)
	(operation::evaluate_for_address, operation::evaluate_for_sizeof):
	New methods.
	* ax-gdb.c (operation::generate_ax): New method.

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