Reset autoloads in response to variable changes

Prior to this fix, autoloads like function and completion autoloads
would check their path variable (like fish_function_path) on every
autoload request. Switch to invalidating it in response to the variable
changing.

This improves time on a microbenchmark:

    for i in (seq 50000)
      setenv test_env val$i
    end

from ~11 seconds to ~6.5 seconds.

src/function.cpp

@@ -339,6 +339,8 @@ int function_get_definition_lineno(const wcstring &name) {
     return 1 + std::count(source.begin(), source.begin() + func_start, L'\n');
 }
 
+void function_invalidate_path() { function_autoloader.invalidate(); }
+
 // Setup the environment for the function. There are three components of the environment:
 // 1. argv
 // 2. named arguments