add some benchmarks

[oscardssmith]

it would be good to add some cpu and gpu benchmarks for a simple broadcast and a matmul/convolution compared to alternatives.

[chriselrod]

I assume you've looked at these: https://github.com/mcabbott/Tullio.jl/tree/master/benchmarks

01 includes some broadcasting, and 02 includes matmuls and permutedims.

[mcabbott]

It would be nice to be more organised about this, ideally both as "advertising" (i.e. to document what it is and isn't good at) and for development / regressions. Locally I have another dozen files like those in benchmarks/01 trying out various examples I came across while writing this, but haven't tried to keep them up to date.