Convert decnum to binary64 (double) instead of decimal64

This is what the JSON spec suggests and will also be less confusing compared to other jq implementations and langauges.

Related to #2939

src/jv.c

@@ -213,7 +213,7 @@ enum {
 #define JVP_FLAGS_NUMBER_LITERAL      JVP_MAKE_FLAGS(JV_KIND_NUMBER, JVP_MAKE_PFLAGS(JVP_NUMBER_DECIMAL, 1))
 
 // the decimal precision of binary double
-#define DEC_NUBMER_DOUBLE_PRECISION   (16)
+#define DEC_NUBMER_DOUBLE_PRECISION   (17)
 #define DEC_NUMBER_STRING_GUARD       (14)
 #define DEC_NUBMER_DOUBLE_EXTRA_UNITS ((DEC_NUBMER_DOUBLE_PRECISION - DECNUMDIGITS + DECDPUN - 1)/DECDPUN)
 
@@ -535,9 +535,13 @@ static decContext* tsd_dec_ctx_get(pthread_key_t *key) {
     }
     else if (key == &dec_ctx_dbl_key)
     {
-      decContextDefault(ctx, DEC_INIT_DECIMAL64);
-      // just to make sure we got this right
-      assert(ctx->digits <= DEC_NUBMER_DOUBLE_PRECISION);
+      // constants suitable for double (binary64)
+      ctx->digits = DEC_NUBMER_DOUBLE_PRECISION;
+      ctx->emax = 1023;
+      ctx->emin = -1022;
+      ctx->round = DEC_ROUND_HALF_EVEN;
+      ctx->traps = 0;
+      ctx->clamp = 1;
     }
     if (pthread_setspecific(*key, ctx) != 0) {
       fprintf(stderr, "error: cannot store thread specific data");

tests/jq.test

@@ -1867,6 +1867,15 @@ false
 1
 1
 
+# decnum to double conversion
+.[] as $n | $n+0 | [., tostring, . == $n]
+[-9007199254740993, -9007199254740992, 9007199254740992, 9007199254740993, 13911860366432393]
+[-9007199254740992,"-9007199254740992",true]
+[-9007199254740992,"-9007199254740992",true]
+[9007199254740992,"9007199254740992",true]
+[9007199254740992,"9007199254740992",true]
+[13911860366432392,"13911860366432392",true]
+
 # abs, fabs, length
 abs
 "abc"