Implement Uni.encode (#5274)

This allows one to write normalized unicode strings. It should be noted
that this is not an optimized implementation at all, written in nqp it
should be faster and even faster if it would be made a syscall.

src/core.c/Uni.pm6

@@ -4,6 +4,7 @@ my class NFKC is repr('VMArray') is array_type(uint32) { ... }
 my class NFKD is repr('VMArray') is array_type(uint32) { ... }
 
 my class X::InvalidCodepoint { ... }
+my class Encoding::Registry { ... }
 
 my class Uni does Positional[uint32] does Stringy is repr('VMArray') is array_type(uint32) {
 
@@ -148,6 +149,17 @@ my class Uni does Positional[uint32] does Stringy is repr('VMArray') is array_ty
         'Uni.new(' ~ self.list.fmt('0x%04x', ', ') ~ ')' ~
             (self.WHAT === Uni ?? '' !! '.' ~ self.^name);
     }
+
+    method encode(Str:D $encoding = 'utf8',
+      :$replacement, Bool() :$translate-nl = False, :$strict --> Blob:D) {
+        my $encoder = Encoding::Registry.find($encoding)
+            .encoder(:$replacement, :$translate-nl, :$strict);
+        my $buf = Buf.new;
+        for self.list -> $character {
+            $buf.append($encoder.encode-chars($character.chr));
+        }
+        $buf;
+    }
 }
 
 my class NFD is Uni {