--- trace_var(varname, hook)
--- trace_var(varname) {|newval| .... }

    Хѿ varname ؤΥեåϿޤ
    varname ʸ Symbol ǻꤷޤ

    ǤΡ֥Хѿפϡüѿ [Ȥ߹ѿ]ޤ᤿`$'
    ǻϤޤѿΤȤǤ

    θƤӽФʹߡvarname ǻꤷХѿ
    ʸޤ Proc ֥ hook
    ɾޤեå Proc ֥Ȥʤ
    ֥å줿ͤϤޤޤեåʣ
    ϿǤޤ

    ȥ졼ˤϡhook  nil 
    ꤹ뤫untrace_var Ѥޤ

    :
        trace_var(:$v) {|val| puts "$v=#{val.inspect}" }
        $v = "foo"   #=> $v="foo"
        $v = 1       #=> $v=1

    hook  nil ʤСꤵƤ
    hook ֤ޤ(֥åϿƤ
    Proc ֥Ȥ֤ޤ)
    ʳϡnil ֤ޤ

