Efficient implementation of `expect` for `TTN`?

`expect(op,state)` with `state` a `TTN` does not appear to be efficient: 

```julia
using ITensors
using ITensorNetworks
s=siteinds("S=1/2",10)
a=randomMPS(s;linkdims=100)
b=random_mps(s;internal_inds_space=100)
expect(a,"Sz") #ITensors implementation
expect("Sz",b) #ITensorNetworks implementation
```

where the last line raises an `OutOfMemory` error.
