Simplify code with `serde_json::from_reader`

[indiv0]

Currently this is not as fast as first loading the response into an in-memory string:

test tests::bench_xkcd           ... bench:      18,558 ns/iter (+/- 5,447)
test tests::bench_xkcd_bufreader ... bench:      58,496 ns/iter (+/- 13,621)
test tests::bench_xkcd_reader    ... bench:     368,452 ns/iter (+/- 145,628)

Where:

• bench_xkcd is the current code (i.e. serde_json::from_str)
• bench_xkcd_reader is parsing a response from a File using serde_json::from_reader
• bench_xkcd_bufreader: same as above, but the File is also wrapped in a BufReader

These results match with the results in this issue: https://github.com/serde-rs/json/issues/160

Seeing as xkcd JSON responses are typically very short, I won't introduce the performance penalty of using BufReader for now.