Apply alternate LinkedList suggested solution.

[mental32]

Motivation

Reference: #2790 (comment)

The current internal LinkedList had an implementation unsuitable for a const-fn constructor due to a generic constraint requiring feature(const_fn) which is nightly-only and we'd like to avoid that.

Applied here is a solution proposed by @hawkw in the referenced comment above that allows LinkedList::const_new to exist without the need of feature(const_fn)

Solution

The LinkedList definition expanded from a single generic parameter T to L, T and an unconstrained impl allowed LinkedList::const_new to be allowed by the compiler and now we have to explicitly specify the Link::Target in the second generic parameter T

[hawkw]

I'm fine with making this change. The new code is definitely a bit uglier than it was previously, but I think we all expected that :) and it's worth it to make the const fn constructors work on stable.

I had a couple small suggestions.

[mental32]

I've applied the suggested changes, I can't see anything else that needs doing here 😄

[hawkw]

this looks great to me — @carllerche, did you want to sign off on it, or shall i just go ahead and ship it?