lib/sh/util.sh: str2varname(): use bash's internal ${var//glob/repl} syntax, which supports character classes ([a-z], etc.), instead of sed (which is slower because it's an external command and uses regexps)

lib/sh/util.sh: added support for caching realpath() using the new shell-variable-based caching. this can be enabled via $realpath_cache and defaults to off because it's currently slower than without. note that the cache needs to be cleared in cd() because relative paths will become invalid.

lib/sh/util.sh: added str2varname() and use it in include_guard_var()

lib/sh/util.sh: added shell-variable-based caching functions

lib/sh/util.sh: canon_rel_path(): use $PWD instead of the slower $(pwd -P) now that $PWD has symlinks expanded

lib/sh/util.sh: expand symlinks in $PWD: moved it before canon_rel_path(), which will require it

lib/sh/util.sh: expand symlinks in $PWD using `cd -P .` so it matches the output of realpath

lib/sh/util.sh: cd(): documented what -P option does and why it's used

lib/sh/util.sh: cd(): always run cd with -P (expand symlinks) so that $PWD will contain a canonical path which matches the output of `readlink -f`. however, don't display the added -P in the logging output because it distracts from the directory being changed to.

lib/sh/util.sh: logging: added log+/-, which take a variable log_level step, and use them in log++/--