Add loader and refactor errors (#917)

This commit adds a `Loader` type, which can be used to load multiple
source strings. This was done to support the work on modules, but
coincidentally enabled consolidating errors, since now `Config::run`
can take a `&Loader`, and in the event of an error, return and `Error`
that borrows from loaded strings. Multiple error types have been
consolidated, and a bunch of ad-hoc error printing was removed.

justfile

@@ -40,7 +40,7 @@ build:
 fmt:
   cargo +nightly fmt --all
 
-watch +COMMAND='test':
+watch +COMMAND='ltest':
   cargo watch --clear --exec "{{COMMAND}}"
 
 man:
@@ -61,7 +61,7 @@ version := `sed -En 's/version[[:space:]]*=[[:space:]]*"([^"]+)"/\1/p' Cargo.tom
 changes:
   git log --pretty=format:%s >> CHANGELOG.md
 
-check: clippy test forbid
+check: clippy fmt test forbid
   git diff --no-ext-diff --quiet --exit-code
   grep '^\[{{ version }}\]' CHANGELOG.md
   cargo +nightly generate-lockfile -Z minimal-versions