Add PrintBuffer classes

[mikee47]

Using ArduinoJson to serialize directly to a file stream performs poorly because writes are done byte-by-byte. This issue is mentioned under performance.

The mentioned ArduinoStreamUtils library is a bit hefty and doesn't particular fit with the existing Sming stream classes.

This PR adds a very simple PrintBuffer class which operates on a Print output and can be employed very easily if additional generic buffering is required.

Note that there is little benefit to using this with other memory-based streams so best left to the library or application to decide when its appropriate.

[what-the-diff]

PR Summary

• New Files Addition
  Three new files have been added namely:
  • WriteBuffer.cpp
  • WriteBuffer.h
  • buffering.rst
• Function Additions in WriteBuffer.cpp
  • A write function has been added that records a single character.
  • An additional write function has been added that records a group of characters.
  • A flush function has been included that relays the saved content to our desired output.
• Class Inclusions in WriteBuffer.h
  • BaseWriteBuffer: A fundamental, basic write-through buffer class.
  • StaticWriteBuffer: A write-through buffer that utilizes stack-based storage.
  • HeapWriteBuffer: A write-through buffer that employs heap storage.
  • DynamicWriteBuffer: A write-through buffer that uses heap storage and possesses its own stream pointer.
• Documentation in buffering.rst
  A thorough documentation has been provided about buffering in Sming along with an explanation and the functionalities of the three buffer classes mentioned previously.