feat(tvix/eval): Sketch out impure builtins

Sketch out a new set of "impure" builtins, which supplement the existing
set of "pure" builtins but are gated behind a feature flag, which allows
them to be omitted by crates depending on tvix-eval that only want pure
evaluation, such as tvixbolt.

Change-Id: I2736017b5c9b4776bbba8758e108ec84887abd66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6655
Reviewed-by: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/builtins/impure.rs

@@ -0,0 +1,7 @@
+use crate::value::Builtin;
+
+/// Return all impure builtins, that is all builtins which may perform I/O outside of the VM and so
+/// cannot be used in all contexts (e.g. WASM).
+pub(super) fn builtins() -> Vec<Builtin> {
+    vec![]
+}