Revert potentially confusing "is default" changes

See https://irclog.perlgeek.de/perl6-dev/2017-07-31#i_14947645

src/core/IO/Handle.pm

@@ -5,9 +5,9 @@ my class Proc { ... }
 my class IO::Handle {
     has $.path;
     has $!PIO;
-    has $.chomp is rw is default(True);
-    has $.nl-in is default(["\x0A", "\r\n"]);
-    has Str:D $.nl-out is rw is default("\n");
+    has $.chomp is rw = Bool::True;
+    has $.nl-in = ["\x0A", "\r\n"];
+    has Str:D $.nl-out is rw = "\n";
     has Str $.encoding;
     has Encoding::Decoder $!decoder;
     has Encoding::Encoder $!encoder;
@@ -191,8 +191,8 @@ my class IO::Handle {
           },
           STORE => -> $, $nl-in {
             $!nl-in = $nl-in;
-            $!decoder && $!decoder.set-line-separators($!nl-in.list);
-            $!nl-in
+            $!decoder && $!decoder.set-line-separators($nl-in.list);
+            $nl-in
           }
         );
     }

src/core/IO/Pipe.pm

@@ -4,8 +4,8 @@ my class IO::Pipe is IO::Handle {
     has $!on-write;
     has $!on-close;
     has $!on-native-descriptor;
-    has $!eof is default(False);
-    has $!closed is default(False);
+    has $!eof = False;
+    has $!closed = False;
 
     method TWEAK(:$!on-close!, :$enc, :$bin, :$!on-read, :$!on-write,
                  :$!on-native-descriptor --> Nil) {

src/core/IO/Socket.pm

@@ -1,8 +1,8 @@
 my role IO::Socket {
     has $!PIO;
-    has Str $.encoding       is default('utf8');
-    has $.nl-in        is rw is default(["\n", "\r\n"]);
-    has Str:D $.nl-out is rw is default("\n");
+    has Str $.encoding = 'utf8';
+    has $.nl-in is rw = ["\n", "\r\n"];
+    has Str:D $.nl-out is rw = "\n";
     has Encoding::Decoder $!decoder;
     has Encoding::Encoder $!encoder;
 
@@ -55,8 +55,10 @@ my role IO::Socket {
             FETCH => { $!nl-in },
             STORE => -> $, $nl-in {
                 $!nl-in = $nl-in;
-                $!decoder && $!decoder.set-line-separators($!nl-in.list);
-                $!nl-in
+                with $!decoder {
+                    .set-line-separators($!nl-in.list);
+                }
+                $nl-in
             }
         )
     }

src/core/IO/Socket/INET.pm

@@ -24,9 +24,9 @@ my class IO::Socket::INET does IO::Socket {
     has Int $.localport;
     has Int $.backlog;
     has Bool $.listening;
-    has $.family is default(PIO::PF_INET);
-    has $.proto  is default(PIO::PROTO_TCP);
-    has $.type   is default(PIO::SOCK_STREAM);
+    has $.family = PIO::PF_INET;
+    has $.proto = PIO::PROTO_TCP;
+    has $.type = PIO::SOCK_STREAM;
 
     my sub split-host-port(:$host is copy, :$port is copy, :$family) {
         if ($host) {

src/core/Proc/Async.pm

@@ -100,9 +100,9 @@ my class Proc::Async {
     has $.path;
     has @.args;
     has $.w;
-    has $.enc          is default('utf8');
-    has $.translate-nl is default(True);
-    has Bool $.started is default(False);
+    has $.enc = 'utf8';
+    has $.translate-nl = True;
+    has Bool $.started = False;
     has $!stdout_supply;
     has CharsOrBytes $!stdout_type;
     has $!stderr_supply;