Symbolic utils extension

[jverzani]

This PR adds an extension for SymbolicUtils. With this loaded, the rewriting rules of SymbolicUtils can be used, which should expand the feature set of SymEngine.jl.

julia> using SymEngine, SymbolicUtils
@
julia> @vars x y
(x, y)

julia> simplify(sin(x)^2 + cos(x)^2)
1

[isuruf]

Looks good. Can you add some tests?

[jverzani]

Ok, borrowed examples from https://symbolicutils.juliasymbolics.org/rewrite/#rule-based_rewriting to do so.

[isuruf]

Thanks