Xaddpy

This pre-release contains implementations using SymEngine as the symbolic core for XADD operations.
We observe a significant performance increase by using SymEngine as opposed to SymPy.
However, the performance noticeably lags behind the original Java version.

From this version, future versions that build on SymEngine will have 0.2.x or up; xaddpy versions that use SymPy will be versioned 0.1.x, which will eventually be deprecated.

For now, there can be bugs or some unstable/unexpected behavior due to the introduction of SymEngine. If you spot any, please create an issue or a pull request if you have a quick fix.

Leaf node hashing via str(expr).

Note that this is not tested fully... May break some functions, but not likely. The reasoning behind this update is that sometimes additional assumptions are added to a leaf expression, which makes the hashing and equality check broken. This has led to duplicated leaf nodes not removed via standardization. Typically, those added assumptions are just redundant and at best not informative. Hence, for now, we deem it alright to just use str(expr) as a unique identifier of a leaf node and its expression.

See the title.

'float' unary op fixed.

DeltaFunctionSubstitution optionally performs reduce_lp only when `is_linear` is set to True.

Supports a new `unary_op`, `float`, that converts leaf nodes to float values.

Bug fixed for `XADD.collect_vars`.