Fallback strtod() when not able to do fast-path

This shall generate best possible precision (if strtod() is correctly
implemented). Need more unit tests and performance tests. May add an
option for accepting precision error. Otherwise LUT in Pow10() can be
reduced.

test/unittest/readertest.cpp

@@ -119,7 +119,9 @@ TEST(Reader, ParseNumberHandler) {
         Reader reader; \
         reader.Parse(s, h); \
         EXPECT_EQ(1u, h.step_); \
-        EXPECT_DOUBLE_EQ(x, h.actual_); \
+        EXPECT_EQ(x, h.actual_); \
+        if (x != h.actual_) \
+            printf("  Actual: %.17g\nExpected: %.17g\n", h.actual_, x);\
     }
 
     TEST_NUMBER(ParseUintHandler, "0", 0);