Rename *string* classes to *str*

Rename for conciseness and improved (Python-inspired) semantics:
•  cstring > istr (immutable string)
•  wstring > mstr (mutable string)
•  wsstring > smstr (statically-allocated mutable string)
•  wdstring > dmstr (dynamically-allocated mutable string)

Rename for conciseness and consistency with the above new names:
•  string_base > str_base
•  _raw_string > _raw_str
•  to_string > to_str
•  *to_string_backend* > *to_str_backend*
•  string_*stream > str_*stream
Change file names containing *string*

Propagate changes from commit 38dc6eaee9ecabe0ab15355a2caf7d506c1c598e to
the names of the files that define the affected classes.