rust: Make workspace more convenient with default

Set the default-members in the top-level `Cargo.toml` file.  This tells
cargo what to do if it is invoked at the top level.

Using this allows builds with the simulate to be done directly from the
top level directory without having to manually cd to the sim directory.

Combined with `bear`, this makes development with vscode much more
handy.

Signed-off-by: David Brown <david.brown@linaro.org>

Author: d3zd3z

Cargo.toml

@@ -2,6 +2,7 @@
 members = ["sim"]
 exclude = ["ptest"]
 resolver = "2"
+default-members = ["sim"]
 
 # The simulator runs very slowly without optimization.  A value of 1
 # compiles in about half the time, but runs about 5-6 times slower.  2