refactor(tvix/eval): split observer traits in two

There are actually two different types of observers, the ones that
observe the compiler (and emitted chunks from different kinds of
expressions), and the ones that trace runtime execution.

Use of the NoOpObserver is unchanged, it simply implements both
traits.

Change-Id: I4277b82674c259ec55238a0de3bb1cdf5e21a258
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6852
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/compiler/mod.rs

@@ -28,7 +28,7 @@ use std::sync::Arc;
 
 use crate::chunk::Chunk;
 use crate::errors::{Error, ErrorKind, EvalResult};
-use crate::observer::Observer;
+use crate::observer::CompilerObserver;
 use crate::opcode::{CodeIdx, Count, JumpOffset, OpCode, UpvalueIdx};
 use crate::value::{Closure, Lambda, Thunk, Value};
 use crate::warnings::{EvalWarning, WarningKind};
@@ -95,7 +95,7 @@ struct Compiler<'observer> {
 
     /// Carry an observer for the compilation process, which is called
     /// whenever a chunk is emitted.
-    observer: &'observer mut dyn Observer,
+    observer: &'observer mut dyn CompilerObserver,
 }
 
 /// Compiler construction
@@ -104,7 +104,7 @@ impl<'observer> Compiler<'observer> {
         location: Option<PathBuf>,
         file: Arc<codemap::File>,
         globals: HashMap<&'static str, Value>,
-        observer: &'observer mut dyn Observer,
+        observer: &'observer mut dyn CompilerObserver,
     ) -> EvalResult<Self> {
         let mut root_dir = match location {
             Some(dir) => Ok(dir),
@@ -1146,7 +1146,7 @@ pub fn compile(
     location: Option<PathBuf>,
     file: Arc<codemap::File>,
     globals: HashMap<&'static str, Value>,
-    observer: &mut dyn Observer,
+    observer: &mut dyn CompilerObserver,
 ) -> EvalResult<CompilationOutput> {
     let mut c = Compiler::new(location, file, globals, observer)?;