When trying to evaluate a Nimble file which imports a Nim module
I was getting the following errors for some reason:
```
/Users/dom/projects/nim/lib/pure/parseopt.nim(229, 46) Error: ambiguous call; both system.paramCount() [declared in /Users/dom/projects/nim/lib/system/nimscript.nim(65, 6)] and os.paramCount() [declared in /Users/dom/projects/nim/lib/pure/os.nim(2613, 8)] match for: ()
```
* Add ability for users to elide ':' or '=' when CLI authors pass a
non-empty partial symbol table. Behavior should be identical to the
old behavior if empty partial symbol tables are passed. "Partialness"
of the symbol table refers to the fact that one need only specify
option keys that are toggles/booleans/do not take arguments, hence
the "NoArg" suffixes in shortNoArg and longNoArg.
commandLineParams() returns seq[TaintedString], so use that consistently
in getopt() and initOptParser(seq[TaintedString]) dropping the taint at
the quoting stage just as with the paramStr() logic.
Fix capitalization inconsistency of cmdLongOption.
Export OptParser.cmd and OptParser.pos so that, at least *in principle*,
users of this API can handle "--" option processing termination or some
"git-like" sub-command stop word with a separate option sub-syntax.
{ Eg., ``case p.key of "": echo "trailing non-option args: ", p.cmd[p.pos..^1]``
or ``case p.kind of cmdArgument: if p.key == "mysubcmd": ...``. } Really,
searching for the last delimiter before p.pos is probably needed to frame
the trailing text..Not the nicest API, but still possible with effort.
* Make requested changes from string to seq[char]
(see https://github.com/nim-lang/Nim/pull/7297)
* Document new behavior and elaborate on some special cases.
* NoArg => NoVal to be less ambiguous/more clear.
* Add more documentation and an example snippet.
* Tweak language. Clarify still using ':'/'=' is ok.
* Add a test case for new NoVal behavior.
