sim: Make `Run` into a proper builder

Rename `Run` to `ImagesBuilder`, and move its definition into the
`images` module.  This makes much more of the functionality local to
this module.  With this locality, all of the fields of `Images` can now
be made private making it easier to add future support for multiple
images.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/tests/core.rs b/sim/tests/core.rs
index 4a1931f..5aa29ae 100644
--- a/sim/tests/core.rs
+++ b/sim/tests/core.rs
@@ -2,7 +2,7 @@
 //!
 //! Run the existing testsuite as a Rust unit test.
 
-use bootsim::{Run, testlog};
+use bootsim::{ImagesBuilder, testlog};
 
 macro_rules! sim_test {
     ($name:ident, $maker:ident, $test:ident) => {
@@ -10,7 +10,7 @@
         fn $name() {
             testlog::setup();
 
-            Run::each_device(|r| {
+            ImagesBuilder::each_device(|r| {
                 let image = r.$maker();
                 assert!(!image.$test());
             });