Eric rightly pointed out in a CR that Semver is...different. Since it
is an abbreviation of the lengthier SemanticVersion name, SemVer seems
more appropriate. Changed.
This change recognizes .yml in addition to the official .yaml extension,
since .yml is actually very commonly used. In addition, while in here, I've
centralized more of the extensions logic so that it's more "data-driven"
and easier to manage down the road (one place to change rather than two).
This change includes some progress on actual compilation (albeit with several
TODOs remaining before we can actually spit out a useful artifact). There are
also some general cleanups sprinkled throughout. In a nutshell:
* Add a compiler.Context object that will be available during template expansion.
* Introduce a diag.Document abstraction. This is better than passing raw filenames
around, and lets us embellish diagnostics as we go. In particular, we will be
in a better position to provide line/column error information.
* Move IO out of the Parser and into the Compiler, where it can cache and reuse
Documents. This will become important as we start to load up dependencies.
* Rename PosRange to Location. This reads nicer with the new Document terminology.
* Rename the mu/api package to mu/schema. It's likely we will need to introduce a
true AST that is decoupled from the serialization format and contains bound nodes.
As a result, treating the existing types as "schema" is more honest.
* Add in a big section of TODOs at the end of the compiler.Compiler.Build function.
