change env_var_t to a vector of strings

Internally fish should store vars as a vector of elements. The current
flat string representation is a holdover from when the code was written
in C.

Fixes #4200

src/parse_execution.cpp

@@ -473,7 +473,9 @@ parse_execution_result_t parse_execution_context_t::run_for_statement(
         }
 
         const wcstring &val = argument_sequence.at(i);
-        env_set(for_var_name, ENV_LOCAL, val.c_str());
+        // This is wrong. It should or in ENV_USER and test if ENV_PERM is returned.
+        // TODO: Fix this so it correctly handles read-only vars.
+        env_set_one(for_var_name, ENV_LOCAL, val);
         fb->loop_status = LOOP_NORMAL;
 
         this->run_job_list(block_contents, fb);