Implement non-fancy functional string type

- support all intrinsic operators for character variables
  (llt, lgt, lle, lge, len, len_trim, trim, adjustl, adjustr,
  index, scan, verify, repeat, char, ichar, iachar)
- support derived type IO (formatted and unformatted)
- implement assignment from character values
- implement constructing string from character values

Author: awvwgk

doc/specs/index.md

@@ -20,6 +20,7 @@ This is and index/directory of the specifications (specs) for each new module/fe
  - [quadrature](./stdlib_quadrature.html) - Numerical integration
  - [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
 
 ## Missing specs