libexpr: Improve lexer performance by using full scanner tables (-Cf)

This trades off some executable size for measurable lexer performance
improvements.

Note on the explicitly enabling 8bit scanner.
This is needed due to the default behavior of flex (excerpt from the manual [1]):

> Flex’s default behavior is to generate an 8-bit scanner unless you
> use the ‘-Cf’ or ‘-CF’, in which case flex defaults to generating
> 7-bit scanners unless your site was always configured to generate 8-bit
> scanners.

Some quantifyable metrics:

Nixpkgs revision: a6e3f45acf4e817532a861ab0eda4ab5485fecc1
Parsing the largest file in nixpkgs: pkgs/development/haskell-modules/hackage-packages.nix.

(Before this patch)

```
$ nix build github:nixos/nix/9fe3077d4#nix-expr
$ du --apparent-size result/lib/libnixexpr.so
2518    result/lib/libnixexpr.so
$ nix build github:nixos/nix/9fe3077d4#nix-cli
$ taskset -c 2,3 hyperfine "GC_INITIAL_HEAP_SIZE=16g \
    result/bin/nix-instantiate --parse               \
    ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix > /dev/null"
  Time (mean ± σ):     375.5 ms ±   6.3 ms    [User: 316.9 ms, System: 56.7 ms]
  Range (min … max):   368.5 ms … 388.3 ms    10 runs
```

(After the patch)

```
$ nix build .#nix-expr
$ du --apparent-size result/lib/libnixexpr.so
2685    result/lib/libnixexpr.so
$ nix build .#nix-cli
$ taskset -c 2,3 hyperfine "GC_INITIAL_HEAP_SIZE=16g \
    result/bin/nix-instantiate --parse               \
    ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix > /dev/null"
  Time (mean ± σ):     326.8 ms ±   4.9 ms    [User: 269.5 ms, System: 55.3 ms]
  Range (min … max):   319.7 ms … 335.5 ms    10 runs
```

Overall, the change is roughly:

- 2518KiB -> 2685KiB ~ 150 KiB of machine code
- 375ms -> 325ms ~ 50ms

The perf uplift for eval-heavy test cases is obviously less noticeable,
but it doesn't make sense not to take this free perf win.

[1]: https://westes.github.io/flex/manual/Options-Affecting-Scanner-Behavior.html#Options-Affecting-Scanner-Behavior

src/libexpr/lexer.l

@@ -1,3 +1,4 @@
+%option 8bit
 %option reentrant bison-bridge bison-locations
 %option align
 %option noyywrap

src/libexpr/meson.build

@@ -112,6 +112,7 @@ lexer_tab = custom_target(
   ],
   command : [
     'flex',
+    '-Cf', # Use full scanner tables
     '--outfile',
     '@OUTPUT0@',
     '--header-file=' + '@OUTPUT1@',