Add utility for iterating over arrays in ERC

Add utility for iterating over arrays in ERC

* lisp/erc/erc-common.el (erc--doarray): Add macro for mapping over
arrays.  ERC has the uncommon requirement of having to repeatedly
traverse strings that contain flags for advertised server features.
It doesn't make sense to translate these meanings into enums or
dynamically generate variables for each flag.  Hash tables and lists
require additional setup and aren't as compact to inspect.
* lisp/erc/erc-dcc.el (erc-dcc-handle-ctcp-send): Use `string-search'
instead of `seq-contains-p' even though performance doesn't matter
here.
* lisp/erc/erc.el (erc--channel-mode-types): Use `erc--doarray'
instead of `dolist'.
(erc--process-channel-modes): Use `erc--doarray' instead of `dolist',
and don't create a string from current char until needed.
(erc--parse-user-modes): Use `erc--doarray' instead of `dolist'.
* test/lisp/erc/erc-tests.el (erc--doarray): New test.  (Bug#67677)