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>
