Improve API for detecting required tools on start

[chshersh]

Iris.need is the main function for checking whether a specified tool executable in PATH:

need :: MonadIO m => [Tool] -> m ()

It supposed to be used like this in the main application code:

app :: App ()
app = Iris.asksCliEnv Iris.cliEnvCmd >>= \case
    Download url -> do
        need ["curl"]
        runDownload url
    Evaluate hs -> do
        need ["ghc", "cabal"]
        runEvaluate hs

Where the Tool type is the following:

iris/src/Iris/Tool.hs

Lines 43 to 71 in c1597ee

	data Tool = Tool
	{ toolName :: Text
	-- ^ @since 0.0.0.0
	, toolSelector :: Maybe ToolSelector
	-- ^ @since 0.0.0.0
	}
	{- |
	@since 0.0.0.0
	-}
	instance IsString Tool where
	fromString :: String -> Tool
	fromString s =
	Tool
	{ toolName = fromString s
	, toolSelector = Nothing
	}
	{- |
	@since 0.0.0.0
	-}
	data ToolSelector = ToolSelector
	{ toolSelectorFunction :: Text -> Bool
	-- ^ @since 0.0.0.0
	, toolSelectorVersionArg :: Maybe Text
	-- ^ @since 0.0.0.0
	}

It does the job but it's a bit suboptimal and maybe inconvenient at times.

I'd love to improve this API in the following way:

• Provide eDSL for specifying the minimum required version
• Provide eDSL for specifying the flag for checking the version

With such an eDSL, it should be possible to write the following code:

ghc :: Iris.Tool
ghc = "ghc" `Iris.versionAtLeast` [8, 10, 7] `Iris.usingVersionFlag` "--numeric-version"

cabal :: Iris.Tool
cabal = "cabal" `Iris.versionAtLeast` [3, 6, 2, 0] `Iris.usingVersionFlag` "--numeric-version"

app :: App ()
app = Iris.asksCliEnv Iris.cliEnvCmd >>= \case
    Download url -> do
        need ["curl"]
        runDownload url
    Evaluate hs -> do
        need [ghc, cabal]
        runEvaluate hs

The checkToolFunction should patched accordingly.

[chshersh]

Part of the API improvements are described in:

• Rethink 'CliEnvSettings' and 'defaultCliEnvSettings' #67 (comment)

[german1608]

I'd like to work on this after #97 is merged, but I don't understand what you're suggesting on the second task. Could you help clarifying it please?