Provide abstract base class for a string object

- provide full compatibility with the string_type
- provide overloaded interfaces in the base class to define all
  character intrinsic functions for both character scalars
  and functional strings
- add preliminary specs on the public API (not developer API)
  of the abstract base class
- add tests to check a minimal implementation of a string class

Author: awvwgk

doc/specs/index.md

@@ -22,6 +22,7 @@ This is and index/directory of the specifications (specs) for each new module/fe
  - [stats](./stdlib_stats.html) - Descriptive Statistics
  - [stats_distribution_PRNG](./stdlib_stats_distribution_PRNG.html) - Probability Distributions random number generator
  - [string\_type](./stdlib_string_type.html) - Basic string support
+ - [string\_class](./stdlib_string_class.html) - Abstract base class for strings
 
 ## Missing specs