Set lexical-binding; Don't explicitly create a new buffer

[akirak]

Setting lexical-binding to t is now a recommended practice for all Emacs packages, so I set it. This spares you from explicitly making a local variable of process-environment.

It is also important to not wrap start-process with a new buffer. If exec-path is set locally in the parent Org file and an iex executable is only available in the local exec-path, you cannot find the executable in the new buffer, which causes an error. Instead, pass the buffer (name) directly to start-process.