Prepare directories to handle --target

[erlanger]

I would suggest to change the directory structure in priv/crates to include the architecture. For example:

priv/crates/x86_64-pc-windows-gnu/foo_nif/
                                 /bar_port/
priv/crates/x86_64-unknown-linux-gnu/foo_nif
                                 /bar_port/

The problem would be to decide which architecture string to use, the erlang one returned by erlang:system_info(system_architecture) or the rust one used by cargo with --target.

The erlang one has the advantage that it would be easier to find the port/nif from the erlang code when it is time to load the nif or open the port.

The rust one is easier to obtain simply from --target but some way needs to be provided to map from the rust architectures to the erlang architecture in order to find the .so or binary at runtime from the erlang environment.

[goertzenator]

Do you mean this to be used only when --target is specified, or for all cases?

[erlanger]

It would simplify the nif/port finding code (in erlang) if it is used for all cases.

[goertzenator]

I don't think that should be default behavior, but a flag to turn that on would be sensible. Perhaps in conjunction with an additional option to build multiple architectures in one go. The reasons to make it non-default are:

• There is at least some existing code that depends on the existing directory structure.
• I've had a complaint that the directory path was already too long.
• Erlang doesn't have a tradition of multiplatform app binaries. Existing C NIFs are not multiplatform, and neither are HIPE binaries.

[erlanger]

I can see the first two points; it's okay to make it optional with a flag to turn it on.