einsum fails in proc
Not sure if I'm doing things correctly. Here is a failing bit of code:
```nim
import arraymancer

proc do_einsum[T](x, y: Tensor[T]): Tensor[T] =
  let r = einsum(x, y):
    x[i, k] * y[i, k]

if is_main_module:
  var a = [[0, 0, 1], [1, 0, 0], [1, 1, 0]].to_tensor.astype(float32)
  var b = [[0, 0, 1], [0, 1, 0], [0, 0, 1]].to_tensor.astype(float32)

  {.hint: "einsum in line".}
  # Compiles and runs fine.
  let r0 = einsum(a, b):
    a[i, k] * b[i, l]
  echo r0

  {.hint: "einsum in proc".}
  # Fails to compile.
  let r1 = do_einsum(a, b)
  echo r1
```
Compiler output:
```
Hint: used config file '/home/u/.choosenim/toolchains/nim-1.6.2/config/nim.cfg' [Conf]
Hint: used config file '/home/u/.choosenim/toolchains/nim-1.6.2/config/config.nims' [Conf]
..............................................................................
/home/u/.nimble/pkgs/nimblas-0.2.2/nimblas/private/common.nim(50, 7) Hint: Using BLAS library with name: libblas.so(||.3|.2|.1|.0) [User]
........................................................................................................................................................
/home/u/.nimble/pkgs/nimlapack-0.2.0/nimlapack.nim(19, 7) Hint: Using LAPACK library with name: liblapack.so(||.3|.2|.1|.0) [User]
....................................................................
/home/u/src/sandbox/nim-arraymancer/bug_einsum_proc.nim(13, 9) Hint: einsum in line [User]
/home/u/src/sandbox/nim-arraymancer/bug_einsum_proc.nim(18, 9) Hint: einsum in proc [User]
stack trace: (most recent call last)
/home/u/.nimble/pkgs/arraymancer-0.7.8/arraymancer/tensor/einsum.nim(543, 47) einsum
/home/u/.nimble/pkgs/arraymancer-0.7.8/arraymancer/tensor/einsum.nim(362, 31) splitLhsRhs
/home/u/.nimble/pkgs/arraymancer-0.7.8/arraymancer/tensor/einsum.nim(329, 14) replaceRhsByContig
/home/u/src/sandbox/nim-arraymancer/bug_einsum_proc.nim(19, 21) template/generic instantiation of `do_einsum` from here
/home/u/src/sandbox/nim-arraymancer/bug_einsum_proc.nim(6, 17) template/generic instantiation of `einsum` from here
/home/u/.nimble/pkgs/arraymancer-0.7.8/arraymancer/tensor/einsum.nim(329, 14) Error: Unsupported kind for `einsum` RHS statement nnkOpenSymChoice
```