Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

core/crypto/hashing_context.cpp

@@ -128,10 +128,6 @@ void HashingContext::_bind_methods() {
 	BIND_ENUM_CONSTANT(HASH_SHA256);
 }
 
-HashingContext::HashingContext() {
-	ctx = nullptr;
-}
-
 HashingContext::~HashingContext() {
 	if (ctx != nullptr)
 		_delete_ctx();