fix crash on startup when luars232 doesn't load

link/serial.fnl

@@ -1,5 +1,5 @@
 ; using https://github.com/srdgame/librs232
-(local rs232 (require :luars232))
+(local (_ rs232) (pcall #(require :luars232)))
 (local command (require "core.command"))
 
 (fn check [err ...]

main.lua

@@ -5,7 +5,7 @@ fv = fennel.view
 pp = function(x) print(fv(x)) end
 lume = require("lib.lume")
 -- these set global variables and can't be required after requiring core.strict
-luars232 = require("luars232")
+_, luars232 = pcall(function () require("luars232") end)
 
 _coroutine_resume = coroutine.resume
 function coroutine.resume(...)