Fix and add some document links

[gemmaro]

This pull request mainly involves modifying links in some document comments.

• cargo-doc and cargo-deadlinks are used for verification.
• Links are specified by name for the corrected parts. Reference: The Rustdoc Book

In addition, there are some corrections that I noticed while modifying the links. The list them is below. If you have any suggestions, please let me know.

• src/parser/mod.rs >
  • Parser, left, right are enclosed in square brackets: I think that linking like this can be not only convenient but also safe since they can be verified that the path is correct.
  • partial_then to then_partial
• src/parser/range.rs > tokens2 to tokens
• src/stream/mod.rs > from_iter to IteratorSteram::new

Also: #262 (I haven't used tools like Travis CI or GitHub Actions yet. It would be nice to have validation like cargo-deadlinks done automatically in the future, but this change does not include the introduction of such tools.)

[gemmaro]

$ echo 'nightly' > rust-toolchain
$ cargo test

passes locally but Rust: nightly fails on Travis CI now (https://travis-ci.org/github/Marwes/combine/jobs/747298381)

[Marwes]

Thanks!