LibCrypto: Change UnsignedBigInteger parser to use a StringView

SignedBigInteger already accepts a StringView; let's avoid the heap
allocation in UnsignedBigInteger.

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp

@@ -67,7 +67,7 @@ size_t UnsignedBigInteger::export_data(Bytes data, bool remove_leading_zeros) co
     return out;
 }
 
-UnsignedBigInteger UnsignedBigInteger::from_base(u16 N, const String& str)
+UnsignedBigInteger UnsignedBigInteger::from_base(u16 N, StringView str)
 {
     VERIFY(N <= 36);
     UnsignedBigInteger result;

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h

@@ -53,7 +53,7 @@ public:
 
     size_t export_data(Bytes, bool remove_leading_zeros = false) const;
 
-    static UnsignedBigInteger from_base(u16 N, const String& str);
+    static UnsignedBigInteger from_base(u16 N, StringView str);
     String to_base(u16 N) const;
 
     u64 to_u64() const;