Bind tai to Instant.tai, rather than assigning

Since Instants are value types, there is no point wasting a Scalar
on each created Instant.  This makes Instant:D.SET-SELF() about 5x
as fast, and saves one Scalar allocation per Instant.

src/core/Instant.pm6

@@ -8,7 +8,8 @@ my class Instant is Cool does Real {
       # Rakudo::Internals.initial-offset. Thus, $.tai matches TAI from 1970
       # to the present.
 
-    method SET-SELF($!tai) { self } # cannot be private because of operators
+    # cannot be private because of operators
+    method SET-SELF(\tai) { $!tai := tai; self }
 
     method new(*@) { X::Cannot::New.new(class => self).throw }