Add support for PCRE2

[mmastrac]

Enable the PCRE2 crate which seems to be significantly faster than onig (3x-8x faster):

With pcre2:

     Running benches/apache.rs (target/release/deps/apache-f30e9c1766c85071)

running 8 tests
test bench_apache_log_match                  ... bench:         450.01 ns/iter (+/- 32.01)
test bench_apache_log_match_anchor           ... bench:         453.31 ns/iter (+/- 29.71)
test bench_apache_log_no_match_end           ... bench:       3,472.72 ns/iter (+/- 457.27)
test bench_apache_log_no_match_end_anchor    ... bench:         665.03 ns/iter (+/- 81.74)
test bench_apache_log_no_match_middle        ... bench:         461.27 ns/iter (+/- 20.25)
test bench_apache_log_no_match_middle_anchor ... bench:         449.36 ns/iter (+/- 22.09)
test bench_apache_log_no_match_start         ... bench:       1,884.20 ns/iter (+/- 121.71)
test bench_apache_log_no_match_start_anchor  ... bench:         249.02 ns/iter (+/- 8.20)

test result: ok. 0 passed; 0 failed; 0 ignored; 8 measured; 0 filtered out; finished in 13.38s

     Running benches/log.rs (target/release/deps/log-1fc987d2bf71525c)

running 4 tests
test bench_log_match                 ... bench:         168.32 ns/iter (+/- 8.42)
test bench_log_match_with_anchors    ... bench:         166.24 ns/iter (+/- 9.23)
test bench_log_no_match              ... bench:         261.31 ns/iter (+/- 20.63)
test bench_log_no_match_with_anchors ... bench:         138.88 ns/iter (+/- 14.65)

test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out; finished in 12.31s

     Running benches/simple.rs (target/release/deps/simple-ef7a70d8b21688bc)

running 4 tests
test bench_simple_pattern_match                ... bench:          78.32 ns/iter (+/- 11.66)
test bench_simple_pattern_match_with_anchor    ... bench:          81.64 ns/iter (+/- 3.53)
test bench_simple_pattern_no_match             ... bench:          78.53 ns/iter (+/- 10.32)
test bench_simple_pattern_no_match_with_anchor ... bench:          73.62 ns/iter (+/- 11.34)

test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out; finished in 4.36s

With onig:

     Running benches/apache.rs (target/release/deps/apache-b74c62eeadec2425)

running 8 tests
test bench_apache_log_match                  ... bench:       1,159.12 ns/iter (+/- 72.14)
test bench_apache_log_match_anchor           ... bench:       1,144.51 ns/iter (+/- 34.72)
test bench_apache_log_no_match_end           ... bench:      22,726.43 ns/iter (+/- 1,024.36)
test bench_apache_log_no_match_end_anchor    ... bench:       4,372.33 ns/iter (+/- 171.75)
test bench_apache_log_no_match_middle        ... bench:       1,151.09 ns/iter (+/- 38.32)
test bench_apache_log_no_match_middle_anchor ... bench:       1,150.05 ns/iter (+/- 47.56)
test bench_apache_log_no_match_start         ... bench:      13,002.12 ns/iter (+/- 491.51)
test bench_apache_log_no_match_start_anchor  ... bench:       1,995.72 ns/iter (+/- 42.47)

test result: ok. 0 passed; 0 failed; 0 ignored; 8 measured; 0 filtered out; finished in 4.69s

     Running benches/log.rs (target/release/deps/log-c28f491c1fb11aff)

running 4 tests
test bench_log_match                 ... bench:         491.83 ns/iter (+/- 68.70)
test bench_log_match_with_anchors    ... bench:         463.17 ns/iter (+/- 44.47)
test bench_log_no_match              ... bench:         760.29 ns/iter (+/- 35.32)
test bench_log_no_match_with_anchors ... bench:         502.67 ns/iter (+/- 44.28)

test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out; finished in 3.19s

     Running benches/simple.rs (target/release/deps/simple-70842287ce6bfb68)

running 4 tests
test bench_simple_pattern_match                ... bench:         187.28 ns/iter (+/- 37.28)
test bench_simple_pattern_match_with_anchor    ... bench:         204.94 ns/iter (+/- 51.67)
test bench_simple_pattern_no_match             ... bench:         106.57 ns/iter (+/- 6.22)
test bench_simple_pattern_no_match_with_anchor ... bench:         101.38 ns/iter (+/- 8.25)