Sisimai::Mail::* Improvement for compatibilities with the Go language version of Sisimai

[azuma, kuniyuki]

• Remove Sisimai::Mail::STDIN->name accessor (not used)
• Remove Sisimai::Mail::Maildir->inodes accessor (not needed to check inode number)
• Make Sisimai::Mail->path point Sisimai::Mail::*->path
• Change Sisimai::Mail->type to Sisimai::Mail->kind
• Call Sisimai::Mail::*->read directly (do not call Sisimai::Mail->read)
• Sisimai::Mail->close is not used
• The Go language version of Sisimai will be released this summer

Accessor/Sisimai::Mail::	Mbox	Maildir	Memory	STDIN
kind(<-type)	o	o	o	o
path	o	o	o	o
dir	o	o
file	o	o
size	o	o	o	o
offset	o	o	o	o
handle	o	o		o
payload			o