Example cargo-embed project using Workspaces

cargo-embed doesn't seem to support workspaces. This simple workspace project is to help debug issue #119.

This project can be built using cargo build from the root directory. However, attempting to use cargo embed fails with:

$ cargo embed
error: package collision in the lockfile: packages common v0.1.0 (C:\path_to_workspace\common) and common v0.1.0 (C:\path_to_workspace\firmware\../common) 
are different, but only one can be written to lockfile unambiguously
       Error Failed to run cargo build: exit code = Some(101)