Merge jethrogb#22

22: Upgrade to nom 5 r=jethrogb a=jonhoo

This patchset builds on jethrogb#21, and bumps the dependency on `nom` to version 5. The change is a fairly major one, since `nom` moved away from macro-based parsers to combinators based on function callbacks.

Fixes jethrogb#19 

---

**Old obsolete text:**

The current state of affairs is that this _almost_ compiles. The biggest remaining issue is how to pass the methods of `expr::PRef` in the new function-passing style. The errors should pretty quickly make it obvious what's going on. I'm honestly not sure what the right way to go about this is. The methods that call them may need to be re-written in the sort of "linear" style that `nom 5` favors. [`nom-methods`](https://docs.rs/nom-methods/) may provide some inspiration here. The new `&mut self` style also makes it awkward to have a combinator that may call multiple different methods on `self`.  It may be that we want to return to the old "consume self return Self" strategy to get around this.

I don't know that I'll have the time to finish this PR up any time soon, so @jethrogb if you want to pick this up and run with it, feel free!

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Co-authored-by: Jon Gjengset <jon@thesquareplanet.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cexpr"
-version = "0.3.6"
+version = "0.4.0"
 edition = "2018"
 authors = ["Jethro Beekman <jethro@jbeekman.nl>"]
 license = "Apache-2.0/MIT"
@@ -13,7 +13,7 @@ keywords = ["C","expression","parser"]
 travis-ci = { repository = "jethrogb/rust-cexpr" }
 
 [dependencies]
-nom = {version = "^4", features = ["verbose-errors"] }
+nom = "5"
 
 [dev-dependencies]
 clang-sys = ">= 0.13.0, < 0.29.0"