REPL namespaces; lib function for pattern matching

Adds support for REPL namespacing, primarily to avoid `main` collisions.
Also adds a library function for an ergonomic pattern matching function
that I've been noodling on. I might explore ways to make list syntax
less annoying specifically for pattern matching like this.

src/Lexer.hs

@@ -59,7 +59,7 @@ keywordT = string "t" *> notFollowedBy alphaNumChar *> pure LKeywordT
 identifier :: Lexer LToken
 identifier = do
   first <- lowerChar <|> char '_'
-  rest  <- many $ letterChar 
+  rest  <- many $ letterChar
               <|> digitChar <|> char '_' <|> char '-' <|> char '?'
               <|> char '$'  <|> char '#' <|> char '@' <|> char '%'
   let name = first : rest