types REFACTOR move decimal64-specific functions into a standalone source file

Split the plugins_types.c file to distinct internal implementation of
plugins types API and the (base) type plugins themselves.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 48c53d1..b8eb844 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -137,6 +137,7 @@
     src/plugins_types_binary.c
     src/plugins_types_bits.c
     src/plugins_types_boolean.c
+    src/plugins_types_decimal64.c
     src/plugins_types_integer.c
     src/plugins_types_string.c
     src/plugins_exts.c