Move staticFor to the inner repo

[mratsim]

Nimble unfortunately removes all folders including tests and benchmarks (and bindings?) on installation. Which is absolutely wrong, especially for libraries tuned to the hardware (bigEndian?).

• [RFC] Nim package structure (library author point-of-view) nim-lang/nimble#472
• Frictionless Nimble directory structures nim-lang/nimble#653
• Nimble package structure RFC nim-lang/Nim#6700

In our case, the libraries doesn't work at all because staticFor is in the helpers/ directory. Move it out.

cc @treeform, cc @guzba

Even nimble needed to use a hack around their recommended src structure

https://github.com/nim-lang/nimble/blob/b629048249ae63117ff90a813037dccab2965983/nimble.nimble#L4-L9

[treeform]

thanks!