Implement pack importing/exporting for downloaded Curseforge packs

Abstract out hash implementations
Implement file saving/downloading

core/hash.go

@@ -0,0 +1,23 @@
+package core
+
+import (
+	"crypto/md5"
+	"crypto/sha256"
+	"crypto/sha512"
+	"errors"
+	"hash"
+)
+
+// GetHashImpl gets an implementation of hash.Hash for the given hash type string
+func GetHashImpl(hashType string) (hash.Hash, error) {
+	switch hashType {
+	case "sha256":
+		return sha256.New(), nil
+	case "sha512":
+		return sha512.New(), nil
+	case "md5":
+		return md5.New(), nil
+	}
+	// TODO: implement murmur2
+	return nil, errors.New("hash implementation not found")
+}