fix(tvix/eval): coerce string interpolation parts to string

With this puzzle piece of string compilation in place, `compile_str`
becomes less redundant, as every part now needs to be compiled the same.
The thunking logic becomes a bit trickier, since we need to thunk even
in the case of `count == 1` if the single part is interpolating.
Splitting the inner (shared) code in a separate function turned out to
be easier for making rustc content.

Change-Id: I6a554ca599926ae5907d7acffce349c9616f568f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6582
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/opcode.rs

@@ -105,6 +105,9 @@ pub enum OpCode {
 
     // Strings
     OpInterpolate(Count),
+    /// Force the Value on the stack and coerce it to a string, always using
+    /// `CoercionKind::Weak`.
+    OpCoerceToString,
 
     // Type assertion operators
     OpAssertBool,