lib: fix overflowing fromHexString tests and example

`fromHexString` is backed by `builtins.fromTOML`. Per [the TOML
v1.0.0 specification]:

> Arbitrary 64-bit signed integers (from −2^63 to 2^63−1) should be
> accepted and handled losslessly. If an integer cannot be represented
> losslessly, an error must be thrown.

[the TOML v1.0.0 specification]: <https://toml.io/en/v1.0.0#integer>

The saturating behaviour of the toml11 version currently used
by Nix is not lossless, and is therefore a violation of the TOML
specification. We should not be relying on it. This blocks the update
of toml11, as it became stricter about reporting this condition.

This, yes, is arguably an evaluation compatibility break. However,
integer overflow was recently explicitly defined as an error by
both Nix and Lix, as opposed to the C++ undefined behaviour it was
previously implemented as:

* <https://nix.dev/manual/nix/stable/release-notes/rl-2.25>
* <https://docs.lix.systems/manual/lix/stable/release-notes/rl-2.91.html#fixes>

This included changing `builtins.fromJSON` to explicitly
reject overflowing integer literals. I believe that the case for
`builtins.fromTOML` is comparable, and that we are effectively testing
undefined behaviour in TOML and the Nix language here, in the same way
that we would have been if we had tests relying on overflowing integer
arithmetic. I am not aware of any use of this behaviour outside of
these tests; the reverted toml11 bump in Nix did not break the 23.11
evaluation regression test, for example.

C++ undefined behaviour is not involved here, as toml11 used the C++
formatted input functions that are specified to saturate on invalid
values. But it’s still a violation of the TOML specification caused
by insufficient error checking in the old version of the library,
and inconsistent with the handling of overflowing literals in the
rest of Nix.

Let’s fix this so that Nix implementations can correctly flag up
this error and we can unblock the toml11 update.

lib/tests/misc.nix

@@ -380,8 +380,9 @@ runTests {
     expected = 255;
   };
 
-  testFromHexStringSecondExample = {
+  # Highest supported integer value in Nix.
-    expr = fromHexString (builtins.hashString "sha256" "test");
+  testFromHexStringMaximum = {
+    expr = fromHexString "7fffffffffffffff";
     expected = 9223372036854775807;
   };
 

lib/trivial.nix

@@ -1114,7 +1114,7 @@ in
     fromHexString "FF"
     => 255
 
-    fromHexString (builtins.hashString "sha256" "test")
+    fromHexString "0x7fffffffffffffff"
     => 9223372036854775807
     ```
   */