`:let` is parsed as NODE_EXCMD (not NODE_LET)

[tyru]

Problem

let

is parsed as

(excmd "let")

Expected

(let)

[tyru]

let var

is also

(excmd "let var")

[tyru]

Currently :const implementation is same #119

[tyru]

let var

should be

(let () var)

because the second argument of assignment :let node is operator.
compiler should not allow arbitrary variable name here (() means nil).

let n = 42

(let = a 42)