Basic `blame` variable functionality

Prints the spans of the source contents for non-incremental variables.

Change-Id: I0ad62d2326591e26414bae69978632816dd3e582
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/build/drydock/+/2464718
5 files changed
tree: 76c753af1471873cc2cb629306d8a1c9e7daba90
  1. src/
  2. .gitignore
  3. Cargo.toml