Public API to increase buffer sizes

The need for Arduino sketches to increase buffer sizes, particularly for HardwareSerial and the Wire library, has been discussed many times.

A mail list thread on May 3-4, 2017 and subsequent github work on a branch on @facchinm's repository ultimately lead to making the RingBuffer class template based: https://github.com/arduino/ArduinoCore-samd/pull/298. While beautiful internally, this is not a public API. Inside HardwareSerial.h, the RingBuffer instance is private. The size is also hidden by a typedef, further obscuring the possibility of increasing the buffer, even from someone so bold as to edit the private class members.

A public API which can actually be used from Arduino sketches & libraries is needed. It needs to be implemented in all Arduino cores, with do-nothing functions in cores which doesn't support variable buffer size, so end users can actually use the public API in their sketches and share their source code with confidence it will at least compile on all other boards.

Previous conversations have involved overloading begin(), perhaps using special classes to represent buffers, and adding functions like addMemory(), addMemoryForWrite(), addStorageForWrite(), addMemoryForRead() to allow users to increase or replace buffer memory at runtime.

But previous work has always stalled far short of actually providing a public API which can actually be used throughout the Arduino ecosystem. A public API which people can actually use without editing the core library or Wire library really is needed.

Start by reviewing HardwareSerial.h, the private RingBuffer instance and typedef, then compare the template-based work in ArduinoCore-samd pull request 298. The design must provide a public buffer-sizing API usable from sketches and libraries across all Arduino cores, with no-op behavior on cores that cannot vary buffer sizes.

Written by the indexing model from the issue text.