lib/sh/util.sh: echo_func(): display where the function was declared (using new func_loc()) instead of where echo_func() was called from. this is more intuitive when debugging, becaues the line # is where the function starts. it also helps remove the dependency on the FUNCNAME/BASH_* arrays, which would produce an inaccurate value if another function call (such as log++) intervened between the caller and echo_func().

lib/sh/util.sh: added func_loc(), which gets where a function was declared in the format file:line, and helper alias set_func_loc

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