refactor(tvix/eval): move spans module to crate root

This is also useful for error-handling related logic, outside of just
the compiler module.

Change-Id: I5c386e2b4c31cda0a0209b31136ca07f00e39e45
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6869
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/lib.rs

@@ -6,6 +6,7 @@ mod eval;
 pub mod observer;
 mod opcode;
 mod source;
+mod spans;
 mod upvalues;
 mod value;
 mod vm;