Loading assets from the cache is significantly slower than from the binary

[Enselic]

Step-by-step:

1. Measure the time it takes bat to highlight examples/simple.rs without cached assets, i.e. when the assets integrated into the binary are used:

% rm -rf ~/.cache/bat
% hyperfine --export-markdown /dev/tty 'bat --color always examples/simple.rs'

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
bat --color always examples/simple.rs	107.4 ± 2.1	104.1	112.5	1.00

2. Create cached assets that are identical to the integrated assets:

% mkdir -p ~/.cache/bat
% echo "---\nbat_version: 0.18.2" > ~/.cache/bat/metadata.yaml
% cp assets/syntaxes.bin ~/.cache/bat/syntaxes.bin
% cp assets/themes.bin ~/.cache/bat/themes.bin

3. Measure again

% hyperfine --export-markdown /dev/tty 'bat --color always examples/simple.rs'

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
bat --color always examples/simple.rs	115.0 ± 2.1	111.9	120.1	1.00

Expected result

There is no significant difference.

Actual result

There is a significant difference.

I will soon create a PR with a proposed fix.