parse: store fileno and lineno in AST

also fix call expr, take call[0] from TUP
keep loc.lineno also in compiler generated ASTs
lineno: add source_file and source_line methods
2 potion_str => PN_STRN
fix call expr change in optional sigs, fix source_file

c24f9a15f886 broke sig_compile for optional args
print \n inside source_string if the lineno changes within
add (unused) potion_source_loc => buf