Stringifier: do not assume associativity of multiplicative (*,/,%,//) operators, add tests

Closes #12

See merge request inducer/pymbolic!42

This reverts commit de093f0b.

Replace Expression.strifier with Expression.make_stringifier

See merge request inducer/pymbolic!41

Replace Expression.strifier with Expression.make_stringifier to allow stringifiers with different constructor arguments

Use python 2,3 compatible syntax for doc strings

See merge request inducer/pymbolic!40

Deprecate if as an identifier

See merge request inducer/pymbolic!38

Parse plus stringify if

See merge request inducer/pymbolic!37

Add list parsing, improve and test tuple parsing

See merge request inducer/pymbolic!36

Minor fix in Slice parsing

See merge request inducer/pymbolic!35

get_dot_dependency_graph(): Give user control over stringification

See merge request inducer/pymbolic!33

Fix use of Indexed for sympy 1.4 (closes #11)

Closes #11

See merge request inducer/pymbolic!34

*statement_stringifier* argument.

The default stringifier escapes double quotes, which are the only
escaped characters in the graphviz language.

This test passes for me on sympy 1.3 as well, so I think the change is
backwards compatible.

Sometimes, it is useful to have labels that include escape sequences
significant to the dot language. The string that comes from repr()
doesn't allow us to do that because it escapes all the backslashes.

>>> print(str("a\lb"))
a\lb

>>> print(repr("a\lb"))
'a\\lb'

This change removes the use of repr() and documents that the
statements are turned into strings using str().

Flip arrows in dep graph

See merge request inducer/pymbolic!32