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>

include/nlohmann/detail/input/parser.hpp

@@ -244,8 +244,9 @@ class parser
     bool sax_parse_internal(SAX* sax)
     {
         // stack to remember the hierarchy of structured values we are parsing
-        // true = array; false = object
-        std::vector<bool> states;
+        // true = array; false = object (bytes, not std::vector<bool>, whose
+        // push_back() is a call with GCC)
+        std::vector<std::uint8_t> states;
         // value to avoid a goto (see comment where set to true)
         bool skip_to_state_evaluation = false;
 
@@ -290,7 +291,7 @@ class parser
                         }
 
                         // remember we are now inside an object
-                        states.push_back(false);
+                        states.push_back(0);
 
                         // parse values
                         get_token();
@@ -315,7 +316,7 @@ class parser
                         }
 
                         // remember we are now inside an array
-                        states.push_back(true);
+                        states.push_back(1);
 
                         // parse values (no need to call get_token)
                         continue;
@@ -432,7 +433,7 @@ class parser
                 return true;
             }
 
-            if (states.back())  // array
+            if (states.back() != 0)  // array
             {
                 // comma -> next value
                 // or end of array (ignore_trailing_commas = true)