Add basic C++ output streaming support

[mikee47]

Add some improvements to the Print class, with support from new String methods.

A full STL stream implementation would require considerable work, with iostream wrappers, etc.

This PR adds basic support to the Print classs for the << insertion streaming operator. Avoiding printf is generally a good idea, and strong C++ types can do a much better job.

I've started going updating the sample projects using this capability to improve code flow and readability, but not yet complete. Let's see what people think before going any further!

C++ streaming operation <<

Building output is commonly done like this::

  Serial.print("Temperature: ");
  Serial.print(temperature);
  Serial.print(" °C, humidity: ");
  Serial.print(humidity);
  Serial.println("%");

We can now do this::

  Serial << "Temperature" << temperature << " °C, humidity: " << humidity << "%" << endl;

Number Printing

Examples::

  Serial.print(12, HEX);         // "c"
  Serial.print(12, HEX, 4);      // "000c"
  Serial.print(12, HEX, 4, '.'); // "...c"
  Serial.print(12);              // "12"
  Serial.print(12, DEC, 4);      // "0012"

Similar extensions are provided for String construction::

  Serial << "0x" << String(12, HEX, 8);       // "0x0000000c"
  Serial << String(12, DEC, 4, '.');          // "..12"

Field-width control

Supported via String methods::

  Serial << String(12).padLeft(4);          // "  12"
  Serial << String(12).padLeft(4, '0');     // "0012"
  Serial << String(12).padRight(4);         // "12  "
  Serial << String(12).pad(-4, '0');        // "0012"
  Serial << String(12).pad(4);              // "12  "

Strongly-typed enumerations

Use of enum class is good practice as it produces strongly-typed and scoped values.
Most of these are also provided with a standard toString(E) function overload.

This allows string equivalents to be printed very easily::

  auto status = HTTP_STATUS_HTTP_VERSION_NOT_SUPPORTED;
  auto type = MIME_HTML;
  Serial.print(type); // "text/html"
  // "Status: HTTP Version Not Supported, type: text/html"
  Serial << "Status: " << status << ", type: " << type << endl;
  // Status: 505, type: 0
  Serial << "Status: " << int(status) << ", type: " << int(type) << endl;

[mikee47]

@slaff I didn't revise all the samples; what are your thoughts on that? Should we complete the job?

[slaff]

what are your thoughts on that? Should we complete the job?

@mikee47 sure, but only if you have time for it.