Extend safe name filter to support Godot 3.4 API

[mivort]

This PR adds 'enum' keyword to rust_safe_name filter and also makes use of this filter for rusty_method_name variable in bindings generator. It allows to build godot-rust with latest stable Godot 3.4 API.

A bit of context: when trying to build bindings for latest stable Godot API, the following errors are produced:

error: expected expression, found keyword `enum`
1972 | ..., self . this . sys () . as_ptr () , class . into () , enum . into () , no_inheritance) ; StringArray :: from_sys (ret) } } # [doc = "...
     |                                                           ^^^^ expected expression

error: expected identifier, found keyword `type`
2302 | ... } # [doc = ""] # [doc = ""] # [inline] pub fn type (& self) -> GodotString { unsafe { let method_bind : * mut sys :: godot_method_bin...
     |                                                   ^^^^ expected identifier, found keyword

It seems that newer API version introduces more keyword collisions which this patch aims to fix.

[Bromeon]

Thank you! Tried it locally, this solves the issue.
There are possibly more steps we need to take for Godot 3.4 compatibility, but they can be handled separately: #814

bors r+

[bors]

Build succeeded:

• full-ci