Get string coercions right

According to @shlevy , there are 2 axes of string coercion:

• coerceToString + coerceMore: Do I coerce integers? Do I recurse into lists?
• addToStore: If coercing a path, do I just convert it into a string, or add it to the store?

There are several string coercion contexts:

• toString: coerceMore = true, addToStore = false
• ${: coerceMore = false, addToStore = true
• derivation argument: coerceMore = true, addToStore = true
• implicit coercion: coerceMore = false, addToStore = false
• forceString: it simply must be a string and nothing else

Shea will double check this.

According to these criteria, we are getting this wrong on toString right now.