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.h

@@ -44,7 +44,7 @@ public:
 	};
 
 private:
-	void *ctx;
+	void *ctx = nullptr;
 	HashType type;
 
 protected:
@@ -57,7 +57,7 @@ public:
 	Error update(PackedByteArray p_chunk);
 	PackedByteArray finish();
 
-	HashingContext();
+	HashingContext() {}
 	~HashingContext();
 };