cperl-mode.el: Use rx sequences for Perl grammar

cperl-mode.el: Use rx sequences for Perl grammar

Following advice by Mattias Engdegård, most uses of rx-to-string
were eliminated, and rx sequences used instead to define Perl
grammar components.
* lisp/progmodes/cperl-mode.el: (cperl-block-declaration-p): New
function, replaces regexp literals.
(cperl-imenu--function-name-regexp-perl): Deleted, use rx
sequences to find imenu entries instead.
(cperl-indent-line): Use rx components instead of regexp literals.
(cperl-sniff-for-indent): use `cperl-block-declaration-p' to
increase accuracy, use rx sequence for labels to replace
inaccurate regexp literals.
(cperl-block-p): Replace inline comment by docstring.  Use
`cperl-block-declaration-p'.
(cperl-after-block-p): Use `cperl-block-declaration-p'.
(cperl-after-block-and-statement-beg): Replace inline comment by
docstring.
(cperl-imenu-package-keywords), (cperl-imenu-sub-keywords),
(cperl-imenu-pod-keywords) : New variables to sort imenu
entries into categories.
(cperl-imenu--create-perl-index): Use rx sequences to collect
imenu entries.
(cperl-init-faces): Use rx components instead of regexp literals
for labels.

* test/lisp/progmodes/cperl-mode-tests.el: Test rx sequences
instead of regexp strings