Avoid using arbitrary `puts` to print output

[st0012]

IRB currently prints most of its output to STDOUT by calling puts directly. This means changing the output target is very difficult (e.g. print to a socket instead, which ruby/debug supports).

[st0012]

I think a possible direction is to:

1. Make InputMethod act as IOMethod. That is, make it responsible for output too (by adding puts and print methods to it?) as Reline/ReadlineInputMethod both already hold @stdout anyway.
2. Change all Kernel#puts invocation to irb_context.io.puts (with some refactors, ofc).