Pair on Optional

[alexander-yevsyukov]

This PR adds a factory method to create Pair using Optional as the second parameter (see #1228).

Other minor changes:

• Fixed style of package description headers. Previously they were H1, as demanded by IDEA style check for Javadoc.
  It turned out that this...
  1) was incorrect from the HTML point of view which assumes only one H1 header per page;
  2) did not meet header nesting in the generated HTML which has package description section as H2.
• Improved code of precondition checks. Each Tuple class now has a companion-like section of code which avoids
  repeating passing the same class name for static precondition checks exposed by the Tuple class.

[codecov]

Codecov Report

Merging #1296 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1296      +/-   ##
============================================
+ Coverage     91.03%   91.07%   +0.04%     
- Complexity     4731     4748      +17     
============================================
  Files           608      608              
  Lines         15063    15088      +25     
  Branches        854      854              
============================================
+ Hits          13712    13741      +29     
+ Misses         1082     1080       -2     
+ Partials        269      267       -2     

[dmdashenkov]

@alexander-yevsyukov, LGTM with a single suggestion (to 4 files).