Rename kcl's Value to Expr
As Jon pointed out, kcl's `Value` enum is actually an expression.
"2+2" isn't a value, it's an expression, which can compute a value.
So I renamed it `Expr`.
Each AST node contains an Option<Digest> which can be optionally set by running compute_digest() over the AST. This will mutate the AST and set the Digest. Fundamentally, the digest is computed from the digest of AST children nodes (using compute_digest) or the explicit raw underlying value.
The underlying hash is changeable by modifying the macro and Digest type alias, and should enable us to determine when an AST -- or fragment of AST -- is the same.
This won't hash the start/end of the AST fragment, so it's, to some extent, position independent. It will hash newlines, comments, etc, which may change in the future depending on how we wind up using this.
Macro for parsing KCL at Rust compile-time
This adds a new `parse_kcl!` macro which takes a string as input. It parses the string into KCL source code _at compile-time_. Invalid KCL will make your Rust project fail to compile.
Fixes https://github.com/KittyCAD/modeling-app/issues/1018
