♻️ Allocate string literals with specific capacity

We know exactly how much memory we're going to need, so we can allocate
this up-front, and save a few malloc/memcpy calls on larger literals.

Author: nevans

lib/net/imap.rb

@@ -3452,7 +3452,8 @@ def get_response_line(buff)
     end
 
     def get_response_literal(buff, literal_size)
-      literal = @sock.read(literal_size) or return
+      literal = String.new(capacity: literal_size)
+      @sock.read(literal_size, literal) or return
       buff << literal
     end