Don't try and remember module path in functions

It doesn't work with inlining and the semantics are too hard to maintain 
when inlining. Update docs to match new (/actual in the case of inlined 
functions) behaviour.

Author: jonathanhogg

docs/language.md

@@ -948,9 +948,12 @@ names](builtins.md) will be defined in a module, but not runtime names (such as
 values in an imported function they must be passed in as arguments. However,
 called functions *do* have access to the current [state](#state).
 
-If a function in a module is called from the main program, and this function
-makes use of any file built-ins – such as `read()` – then these will load files
-relative to the *module* file.
+During execution of an imported module, any calls to [file
+built-ins](builtins.md#file-functions) – such as `read()` – will resolve
+filenames relative to the path of the module. However, if a function in a module
+is called from the main program, and that function calls any file built-ins,
+then these will resolve filenames relative to the path of the main program, not
+the module.
 
 Modules may import from other modules, provided that a cycle is not created: if
 a module attempts to import a module further up the current import chain, then

src/flitter/language/vm.pyx

@@ -484,7 +484,6 @@ cdef class Function:
     cdef readonly tuple defaults
     cdef readonly Program program
     cdef readonly int64_t address
-    cdef readonly object root_path
     cdef readonly bint record_stats
     cdef readonly tuple captures
     cdef readonly int64_t call_depth
@@ -501,7 +500,6 @@ cdef class Function:
             return null_
         cdef int64_t i, lnames_top, stack_top, k=PyTuple_GET_SIZE(self.captures), m=PyTuple_GET_SIZE(self.parameters), n=PyTuple_GET_SIZE(args)
         cdef PyObject* objptr
-        cdef object saved_path
         if context.state is None:
             context.state = StateDict()
         cdef VectorStack lnames = context.lnames
@@ -522,8 +520,6 @@ cdef class Function:
                 push(lnames, <Vector>objptr)
             else:
                 push(lnames, <Vector>PyTuple_GET_ITEM(self.defaults, i))
-        saved_path = context.path
-        context.path = self.root_path
         self.call_depth += 1
         try:
             self.program._execute(context, self.address, self.record_stats)
@@ -533,7 +529,6 @@ cdef class Function:
         cdef Vector result = pop(stack)
         assert stack.top == stack_top, "Bad function return stack"
         assert lnames.top == lnames_top, "Bad function return lnames"
-        context.path = saved_path
         return result
 
 
@@ -1386,7 +1381,6 @@ cdef class Program:
                     function.parameters = (<InstructionFunc>instruction).parameters
                     function.program = self
                     function.address = (<InstructionFunc>instruction).offset + pc
-                    function.root_path = context.path
                     function.record_stats = record_stats
                     n = PyTuple_GET_SIZE(function.parameters)
                     function.defaults = pop_tuple(stack, n)