chore(release): bump to 0.15

[jeertmans]

Release 0.14.3 as 0.15, as it bumped the MSRV, see #449.

[github-actions]

Benchmark results with default features:

group                                         default_before                         default_changes
-----                                         --------------                         ---------------
count_ok/identifiers                          1.13    881.4±3.87ns   842.8 MB/sec    1.00   783.3±10.15ns   948.4 MB/sec
count_ok/keywords_operators_and_punctators    1.04      2.9±0.07µs   703.8 MB/sec    1.00      2.8±0.06µs   735.4 MB/sec
count_ok/strings                              1.00    526.4±5.11ns  1577.9 MB/sec    1.38   726.2±10.71ns  1143.8 MB/sec
iterate/identifiers                           1.06    882.1±4.42ns   842.2 MB/sec    1.00   834.5±33.52ns   890.2 MB/sec
iterate/keywords_operators_and_punctators     1.00      2.5±0.02µs   808.1 MB/sec    1.13      2.8±0.05µs   715.1 MB/sec
iterate/strings                               1.00   529.1±10.27ns  1569.9 MB/sec    1.40    739.8±4.63ns  1122.8 MB/sec

Benchmark results with feature "forbid_unsafe":

group                                         forbid_unsafe_before                   forbid_unsafe_changes
-----                                         --------------------                   ---------------------
count_ok/identifiers                          1.05   940.8±18.19ns   789.7 MB/sec    1.00   897.2±11.90ns   828.0 MB/sec
count_ok/keywords_operators_and_punctators    1.04      2.8±0.05µs   714.6 MB/sec    1.00      2.7±0.04µs   744.5 MB/sec
count_ok/strings                              1.03    589.4±9.97ns  1409.2 MB/sec    1.00   573.5±10.66ns  1448.4 MB/sec
iterate/identifiers                           1.04   934.2±34.22ns   795.3 MB/sec    1.00   896.5±12.13ns   828.7 MB/sec
iterate/keywords_operators_and_punctators     1.06      3.0±0.07µs   678.4 MB/sec    1.00      2.8±0.02µs   716.9 MB/sec
iterate/strings                               1.01    584.0±7.47ns  1422.4 MB/sec    1.00    575.3±4.47ns  1443.8 MB/sec