Replace vtable dispatch with switch-case in match_all_states and unwind (up to ~2x speedup)

[e4d08]

The perl_matcher's match_all_states() and unwind() used static vtables of member function pointers (s_match_vtable[34] and s_unwind_table[19]) to dispatch on the current state type. This replaces them with a direct switch-case dispatch, which enables better branch prediction.

Changes

• Replaced the static matcher_proc_type s_match_vtable[34] array with a switch(pstate->type) in the main match loop (perl_matcher_non_recursive.hpp (include/boost/regex/v5/perl_matcher_non_recursive.hpp))
• Replaced the static unwind_proc_type s_unwind_table[19] array with a switch(m_backup_state->state_id) in unwind()
• Removed the unwind_proc_type typedef from perl_matcher.hpp
• Replaced magic number literals in saved_state construction (15, 16, 17, 18) with named enum values (saved_state_recursion_pop, saved_state_commit, saved_state_then, saved_state_change_case).

Performance effect

Benchmarked with the tests from ./performance regex_perf.cpp on Intel Core i3-4005U CPU @ 1.70GHz.

Test	vtables	switch
Literal abc (extended)	3.468 ms	2.888 ms (1.20x)
Email address validation (perl)	12.761 ms	7.858 ms (1.62x)
Simple alternation Beman|John|Dave (perl)	79.611 ms	72.322 ms (1.10x)
HTML <p> tag, case-insensitive (perl)	313.001 ms	207.437 ms (1.51x)
HTML <a> anchor tag (extended)	1,545.587 ms	1,266.425 ms (1.22x)
HTML <a> anchor tag, case-insensitive (perl)	502.679 ms	357.452 ms (1.41x)
HTML <font> tag with contents, case-insensitive (perl)	364.229 ms	217.004 ms (1.68x)
C++ syntax highlighting pattern (perl)	92,775.666 ms	44,371.532 ms (2.09x)
C++ class/struct declaration (perl)	19,326.816 ms	14,857.844 ms (1.30x)
Boost #include directives (perl)	2,259.924 ms	1,922.954 ms (1.18x)

Run with:

g++ -std=c++17 -O3 regex_perf.cpp -o regex_perf
taskset --cpu-list 1 ./regex_perf