Tidy up some configuration options.

MP_ALLOC_* -> MICROPY_ALLOC_*
MICROPY_PATH_MAX -> MICROPY_ALLOC_PATH_MAX
MICROPY_ENABLE_REPL_HELPERS -> MICROPY_HELPER_REPL
MICROPY_ENABLE_LEXER_UNIX -> MICROPY_HELPER_LEXER_UNIX
MICROPY_EXTRA_* -> MICROPY_PORT_*

See issue #35.
diff --git a/py/scope.c b/py/scope.c
index ab29af7..839e821 100644
--- a/py/scope.c
+++ b/py/scope.c
@@ -71,7 +71,7 @@
     }
     scope->raw_code = mp_emit_glue_new_raw_code();
     scope->emit_options = emit_options;
-    scope->id_info_alloc = MP_ALLOC_SCOPE_ID_INIT;
+    scope->id_info_alloc = MICROPY_ALLOC_SCOPE_ID_INIT;
     scope->id_info = m_new(id_info_t, scope->id_info_alloc);
 
     return scope;
@@ -92,8 +92,8 @@
 
     // make sure we have enough memory
     if (scope->id_info_len >= scope->id_info_alloc) {
-        scope->id_info = m_renew(id_info_t, scope->id_info, scope->id_info_alloc, scope->id_info_alloc + MP_ALLOC_SCOPE_ID_INC);
-        scope->id_info_alloc += MP_ALLOC_SCOPE_ID_INC;
+        scope->id_info = m_renew(id_info_t, scope->id_info, scope->id_info_alloc, scope->id_info_alloc + MICROPY_ALLOC_SCOPE_ID_INC);
+        scope->id_info_alloc += MICROPY_ALLOC_SCOPE_ID_INC;
     }
 
     // add new id to end of array of all ids; this seems to match CPython