convenience class symbolic ExpressionTreeWalker(Converter)

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This class can already be used as base class for SubstituteFunction but is intended also for other uses.

sage: from sage.symbolic.expression_conversions import ExpressionTreeWalker
sage: from sage.symbolic.random_tests import random_expr
sage: foo = random_expr(20, nvars=2)
sage: s = ExpressionTreeWalker(foo)
sage: bool(s() == foo)
True

Used in #9424, #14801, #17849

Depends on #18255

CC: @kcrisman @nilesjohnson

Component: symbolics

Author: Ralf Stephan

Branch/Commit: fa73285

Reviewer: Marc Mezzarobba

Issue created by migration from https://trac.sagemath.org/ticket/17759

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Branch: public/17759

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Commit: 3781eec

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New commits:

3781eec

17759: convenience class symbolic ExpressionTreeWalker(Converter)

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Description changed:

--- 
+++ 
@@ -8,3 +8,4 @@
 sage: bool(s() == foo)
 True
 ```
+Used in #9424

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Description changed:

--- 
+++ 
@@ -8,4 +8,4 @@
 sage: bool(s() == foo)
 True
 ```
-Used in #9424
+Used in #9424, #14801

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comment:7

OK, this is not really a tree walker but a deep copier, i.e., function objects are called again. This misses possible hold arguments that were given but are lost, see #17849. So let's implement a real tree walker in this ticket that only replaces specific operators but leaves the rest of the tree as is.

EDIT:

• http://www.ginac.de/tutorial/#Visitors-and-tree-traversal
• http://www.ginac.de/tutorial/#Applying-a-function-on-subexpressions

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Work Issues: handle hold=True,hypergeometric

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Branch pushed to git repo; I updated commit sha1. New commits:

9375510	Merge branch 'develop' into t/17759/public/17759
16aa81d	17759: handle hold=True and hypergeometric