psi_parse: Add skeleton psi_dostring method
psi_parse: Add private macro for checking parens

_psi_openp checks for an opening parenthesis
_psi_closep checks for a closing parenthesis

These function-like macros are undef'd at the end of the file
psi_parse: Define size of working memory buffer

When parsing files, the contents will be read in chunks then fed
to the parser.
psi_parse: Add psi_dofile function

Will load contents of file into a working buffer and parse them
using psi_dostring