Fix functions relating YaTeX-beginning-of-environment or ...

YaTeX-end-of-environment.
Line indentation by TAB much improved.
Functions that work at enclosing environments, YaTeX-enclose-<ENVNAME>,
introduced.  Functions for enclosing verbatim and equations are supplied.
SPC, DEL, +, - in YaTeX-hierarchy buffer.
Compensate odd highlighting of hilit19.

Set YaTeX-indent-line to 'indent-line-function. ...

Revise fill features.