feat(tvix/eval): implement opcode for function calls in VM

Nix functions always have a single argument and we do not yet make
efforts to optimise this in Tvix for known multi-argument functions
being directly applied.

For this reason, the call instruction is fairly simple and just calls
out to construct a new call frame.

Note that the logic for terminating the run loop has moved to the top
of the dispatch; this is because the loop run needs to be skipped if
the call frame for the current lambda has just been dropped.

Change-Id: I259bc07e19c1e55cd0a65207fa8105b23052b967
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6249
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -32,6 +32,9 @@ pub enum ErrorKind {
     // Unknown variable in dynamic scope (with, rec, ...).
     UnknownDynamicVariable(String),
 
+    // Attempt to call something that is not callable.
+    NotCallable,
+
     ParseErrors(Vec<rnix::parser::ParseError>),
 
     AssertionFailed,