How to make code GC-Safe when a global constant is used in a proc ?

[jbcomps]

When cloning the repo in question, changing the `let` to `const`, and building 
with nim version `1.4.0` and `1.5.1 (devel)`, I get the following. No errors.

test 1 passed test 2

c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
    passed