Arraymancer is bugged with gc:arc
Arraymancer seems to be bugged with --gc:arc.

I encouter blocking code / segfault on Tensor operation.
The following code allows to reproduce with --gc:arc:
```
import arraymancer

proc doubleMe(data : Tensor[float]): Tensor[float]=
  result = data*2

proc squareMe(data : Tensor[float]): Tensor[float]=
  result = data*.data

let A : seq[float] = @[0.11, 0.12, 0.13, 0.14]
echo A.type
echo A
var B : Tensor[float] = A.toTensor
echo B
# Blocking
var C = doubleMe(B)
echo C
#Segfault
var D = squareMe(C)
echo D
```

It works fine with any other gc. 

I installed arraymancer with `nimble install arraymancer@#head`

Nim version is latest devel : 
Nim Compiler Version 1.1.1 [Linux: amd64]
Compiled at 2020-03-05
Copyright (c) 2006-2019 by Andreas Rumpf