[spec] Add Fixed to spec (#1456)

* [spec] Add Fixed to spec

* Fixes #1195
* Define type & parameters
* Add Fixed as argument type to type conversions
* Add Fixed as argument type to relevent PrimOps (with link to tables)
* Add asFixed PrimOp
* Add IncP/DecP/SetP primops
* Add fixed-point width/point propagation tables

* Update spec pdf

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>