`no_std` + `alloc` Template Gen

[pinkforest]

• Mostly the error type
• alloc via feature that extern crate alloc
• heapless via feature that brings heapless types
• std is usually a default feature (when needed) or usually non-default that can be activated for std error type
• error type is making it's way into core though and there is also core-error crate

e.g.

• khonsulabs/justjson@968b786
• khonsulabs/justjson@da7c03c
• khonsulabs/justjson@330dbd6
• khonsulabs/justjson@3465485
• https://github.com/fission-codes/hexxo/commit/982ac3ceb44dc50abd4e7be96c34f75d589b0318

Also thiserror is simple to replace out

• khonsulabs/justjson@70765d9

[zeeshanlakhani]

Yep. A no_std variant with heapless would be nice, via prompt.