Print the value in error: cannot coerce messages (#9553)

* Print the value in `error: cannot coerce` messages

This extends the `error: cannot coerce a TYPE to a string` message
to print the value that could not be coerced. This helps with debugging
by making it easier to track down where the value is being produced
from, especially in errors with deep or unhelpful stack traces.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/manual/rl-next/print-value-in-coercion-error.md

@@ -0,0 +1,50 @@
+synopsis: Coercion errors include the failing value
+issues: #561
+prs: #9553
+description: {
+
+The `error: cannot coerce a <TYPE> to a string` message now includes the value which caused the error.
+
+Previously, a failed string coercion produced a confusing error message if the trace didn't show where the offending value was defined:
+
+```bash
+$ nix-instantiate --eval --expr '
+let x = { a = 1; }; in
+
+"${x}"
+'
+error:
+       … while evaluating a path segment
+
+         at «string»:4:2:
+
+            3|
+            4| "${x}"
+             |  ^
+            5|
+
+       error: cannot coerce a set to a string
+```
+
+Now, the error message includes the value itself:
+
+```bash
+$ nix-instantiate --eval --expr '
+let x = { a = 1; }; in
+
+"${x}"
+'
+error:
+       … while evaluating a path segment
+
+         at «string»:4:2:
+
+            3|
+            4| "${x}"
+             |  ^
+            5|
+
+       error: cannot coerce a set to a string: { a = 1; }
+```
+
+}