lift the post-processing tools (viewer, csv, decimate) into their own crate

- these tools shouldn't need access to coremem internals.
- lifting them out reduces some dependencies in coremem-the-library.
- separation allows faster iteration in the coremem library while
  temporarily breaking the post-processing tools (specifically,
  those tools could take deps on a specific coremem version and thereby
  we split the update process into two, smaller steps).

crates/coremem/src/lib.rs

@@ -11,7 +11,6 @@ pub mod driver;
 pub mod geom;
 pub mod mat;
 pub mod meas;
-pub mod post;
 pub mod real;
 pub mod render;
 pub mod sim;