Replaced printTo() with serializeJson()

* Added `serializeJson()` and `serializeJsonPretty()`
* Added `measureJson()` and `measureJsonPretty()`
* Removed `printTo()` and `prettyPrintTo()`
* Removed `measureLength()` and `measurePrettyLength()`

test/JsonVariant/CMakeLists.txt

@@ -2,13 +2,12 @@
 # Copyright Benoit Blanchon 2014-2018
 # MIT License
 
-add_executable(JsonVariantTests 
+add_executable(JsonVariantTests
 	as.cpp
 	compare.cpp
 	copy.cpp
 	is.cpp
 	or.cpp
-	printTo.cpp
 	set_get.cpp
 	subscript.cpp
 	success.cpp