Match literals without a stack array and keep parser states in bytes

scan_after_whitespace() built the expected text of true, false, and null in
local std::array objects. With -fstack-protector-strong (the default of
Ubuntu's GCC, among others), a local array gives the function a stack canary
check, which then ran on every token. scan_literal() now takes the text
after the first character as a string literal, with its length as a template
parameter; for contiguous input it compares the rest at once and falls back
to the byte loop on a mismatch, so errors are reported at the same
character. It is out of line, which keeps the token dispatch small.

The parser kept the nesting of arrays and objects in a std::vector<bool>,
whose push_back() GCC calls out of line; it is a std::vector<std::uint8_t>
now.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-stream-bulk-scan.cpp

@@ -171,7 +171,8 @@ std::vector<std::string> corpus()
         R"("unterminated)", "\"control\x01character\"", "\"ill-formed \xc3\x28 UTF-8\"", R"("bad \x escape")",
         R"("\ud800 lone surrogate")",
         // literals
-        "true", "false", "null", "tru", "nul", "falsey",
+        "true", "false", "null", "tru", "nul", "falsey", "[true,false,null]", "nulL", "truex", "[fals e]", "t",
+        "true1", "null null", "[false]",
         // structures
         "[]", "{}", "[1,2,3]", R"({"a":1,"b":[true,false,null],"c":{"d":"e"}})", "[1,]", R"({"a" 1})", "[[[[[]]]]]",
         // pretty-printed