Fix exponentionality in Clean.Types.Parse
Previously, the time and heap needed for these start rules would grow exponentionally:
Start = parseType ['[(Int,{#Char},T)] -> [(Int,String,T)]']
Start = parseType ['[(Int,{#Char},T)] -> [(Int,String,T,T)]']
Start = parseType ['[(Int,{#Char},T)] -> [(Int,String,T,T,T)]']This was due to pure [] <|> optContext in addContextAsConstFunction. This function adds context to a type which is not a function type (e.g., a | toString a), but should only be used when it cannot be added to a parent function type (e.g., a -> b | toString a must be parsed as ~Func [a] b [toString a] rather than ~Func [a] (Func [] b [toString a]) []; hence the pure [] <|> ....
The solution taken here is to add a pPeek which returns the current token stream, and have addContextAsConstFunction return immediately if the next token is not |.