Major PRs in the run up to Lua53

[TerryE]

* Includes Updates *
This doesn't really fall into the Bugrep or Feature request, but instead aims to provide a roadmap of major PRs leading to the deployment of Lua53. This roadmap currently only covers the ESP8266 platform but I fully intend to slipstream a parallel exercise for the ESP32 soon thereafter. It is first worthwhile reiterating a key implementation goal that constrains and colours this roadmap:

• The app/lua and app/lua53 directory hierarchies will contain the variant Lua versions within the same git branch. All other directories will be common to both Lua versions.

A corollary to this is that I will need to do a series of PRs prior to releasing any lua53 version to make our current lua(51) version compatible with this unified interface. These PRs will deploy additional functions and features to our current version. This current (as at 2nd July) plan to do the following PRs:

1. c_ prefix removed (PR Major cleanup - c_whatever is finally history. #2838) thanks to Johnny.
   
   We will then break to flush outstanding PRs and do the next master drop, with further commits to dev after this.

2. Input and output handllng (RFC - Sorting out the input handling #2787 + PR Lua 5.1 to 5.3 realignement phase 1 #2836). A new pipe module adds standard pipe functionality; tasking moved into the core VM; rewrite of lua.c (basically the lua53 version backported to the lua51 API) to use pipes and two standard pipes, stdin and stdout; new call wrapper to provide automatic panic capture.

3. Rename app directory hierarchy to align this to the esp32 hierarchy. (See RFC Why have different coding standards for ESP32 and EP8266 modules? #2811) As well as the split of modules into the core, common and esp8266 modules directories, most of this work is Makefile magic and file moves. This initial version will focus on changes to the core modules. I will also initially introduce a legacy modules directory which contains the remaining modules. This last legacy group is for those modules which have not yet been converted to a Lua version neutral coding standard. Lua (51) builds will include the legacy modules but Lua53 ones will not.

4. Move commonly used modules out of the legacy category. This is so the first deployment of lua53 will have a reasonable rich set of modules supported.

5. First version of lua53.

Each of these PRs is going to be big. We will need a minimum of two weeks between each for review and feedback before merging, so we might achieve an overall progress of 1 per month.

I will update this OP with and changes agreed in the following discussion.

[TerryE]

Addressed in #3075