Fix word boundary assertions under C++20

[BigRedEye]

In C++20 with P1614R2 implemented, zero-width assertions \b and \B do not work. For example, the following code fails with "Pattern can never match":

#include <hyperscan/src/hs.h>

#include <cstdio>


int main() {
    hs_compile_error_t* err = nullptr;
    hs_database_t* db = nullptr;
    hs_platform_info_t platform;
    hs_populate_platform(&platform);
    hs_error_t res = hs_compile("foo\\b", 0, HS_MODE_BLOCK, &platform, &db, &err);
    if (res != HS_SUCCESS) {
        printf("Error: %s\n", err->message);
    }
    hs_free_database(db);
}

This is caused by breaking change in std::pair comparison operators (after P1614R2 std::pair uses operator<=>). The spaceship operator prefers implicit conversion to bool, if exists:
https://godbolt.org/z/xnPreToW4
https://godbolt.org/z/T4cKEPrTf

So nearly all values of type std::pair<ue2::graph_detail::vertex_descriptor, ue2::graph_detail::vertex_descriptor> compare equal: https://github.com/intel/hyperscan/blob/master/src/util/ue2_graph.h#L179-L188, so edge_cache_t is malformed: https://github.com/intel/hyperscan/blob/master/src/compiler/asserts.cpp#L284-L288, and the NFA graph after removeAssertVertices becomes disconnected.

The fix is simple: just mark operator bool() as explicit.

[higher-performance]

Hi, I believe the same problem exists here as well:

hyperscan/src/util/ue2_graph.h

Line 218 in f2fdb3b

operator bool() const { return p; }