feat(tvix/eval): implement builtins.fromTOML

This allows parsing TOML from Tvix. We can enable the eval-okay-fromTOML
testcase from nix_tests. It uses the `toml` crate, and the serde
integration it brings with it.

Change-Id: Ic6f95aacf2aeb890116629b409752deac49dd655
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7920
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/Cargo.toml

@@ -27,6 +27,7 @@ serde_json = "1.0"
 smol_str = "0.1"
 tabwriter = "1.2"
 test-strategy = { version = "0.2.1", optional = true }
+toml = "0.6.0"
 xml-rs = "0.8.4"
 
 [dev-dependencies]
@@ -42,7 +43,7 @@ git = "https://github.com/JamesGuthrie/test-generator.git"
 rev = "82e799979980962aec1aa324ec6e0e4cad781f41"
 
 [features]
-default = [ "impure", "arbitrary", "nix_tests", "backtrace_overflow" ]
+default = ["impure", "arbitrary", "nix_tests", "backtrace_overflow"]
 
 # Enables running the Nix language test suite from the original C++
 # Nix implementation (at version 2.3) against Tvix.
@@ -52,10 +53,10 @@ nix_tests = []
 impure = []
 
 # Enables Arbitrary impls for internal types (required to run tests)
-arbitrary = [ "proptest", "test-strategy", "imbl/proptest" ]
+arbitrary = ["proptest", "test-strategy", "imbl/proptest"]
 
 # For debugging use only; not appropriate for production use.
-backtrace_overflow = [ "backtrace-on-stack-overflow" ]
+backtrace_overflow = ["backtrace-on-stack-overflow"]
 
 [[bench]]
 name = "eval"