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Add the unary minus #26

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@user-undeclared user-undeclared commented Oct 31, 2023

a unary minus can be thought of as a subtraction with only a right hand side expression (so -1 can be thought of as None - 1. because of that, to implement the unary minus, the lhs field of the Op in Expr was made optional

the unary minus was added by adding a new Expr and enum called UnOp
(short for Unary Operator). `parse_binary_operator` was also renamed
to `parse_impl` since it now parses both binary and unary operators
adding unary operators to the pattern matching code was easier than i
thought! thanks zozi for the gud cood

the unary minus ended up being pretty intuitive to use, but there are
some things that are both weird to use and hard to fix. for instance, by
making it so -1^2 = -(1^2), it also makes it so -1 + 2 = -(1 + 2). i
already tried to fix that but it turned into a mess
so it turns out that making the unary minus part of `Op` actually works
better than putting it into a separate `UnOp` and makes it easier to
give it the correct precedence
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