feat(tvix/eval): introduce mechanism for defining builtins

Adds a new builtins module in which builtins can be constructed. The
functions in this module should return a correctly structured value to
be passed to the compiler's `globals`.

This is wired up all the way to the compiler with an example
`toString` builtin, available as a global. Note that this does not yet
actually behave like the real toString, which has some differences
from `Display`.

Change-Id: Ibb5f6fbe6207782fdf2434435567fc1bd80039a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6254
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/lib.rs

@@ -1,3 +1,4 @@
+mod builtins;
 mod chunk;
 mod compiler;
 mod errors;