FEATURE: add support for input/instance in .s files

[buggsley]

Allow a file to have multiple include statements saving the LPN, and
restarting as it hits each one.

Allow a file to have an input statement that reads the file and
processes as if it appeared inline where the input statement is.

Allow a file to have an instance statement that takes an instance name
and a .s filename and renames all local elements using the instance
name then inlines the file. To support this, will need a list of
global names at the top of the .s file.