Fix String deprecated methods

[nukosuke]

Emacs	27.0.50 build 10
Elixir	1.8.1-otp-21
M-x alchemist-version	Alchemist version: 1.8.2

When I run alchemist-mix (C-C a x), I got some warnings like this

This patch changes as bellow.

• Use String.trim/1 instead of String.strip/1
• Use String.trim_trailing/1 instead of String.rstrip/1
• Use String.to_charlist/1 instead of String.to_char_list/1