LWT 5 support (links-lang#804)

* Lwt 5.* wont compile

Resolved by installing conf-libev as suggested by ocsigen/lwt#760

* Nested calls to `Lwt_main.run`

Lwt 5.* prohibits nested calls of `Lwt_main.run`. Previously every call to `Proc.run` would invoke `Lwt_main.run` which could result in nested calls. To resolve this `Proc.run` no longer calls `Lwt_main.run` rather we now have a function `start : (unit -> 'a Lwt.t) -> 'a` which is called once when the evaluator starts. This requires a change of types for `Proc.run` such that it returns a `Lwt` computation, meaning its signature is now `(unit -> 'a Lwt.t) -> 'a Lwt.t`. As a consequence it is necessary to move `value` from `evalir.ml` into the Lwt monad, because `Proc.atomically` (used in `ApplyPure`) was re-calling the main LWT entry point.

Co-authored-by: Simon Fowler <[email protected]>

.travis.yml

@@ -6,11 +6,12 @@ services:
 before_install:
   - sudo add-apt-repository -y ppa:avsm/ppa
   - sudo apt-get -qq update
-  - sudo apt-get install -y ocaml-nox ocaml-native-compilers python3-setuptools python3-pip
+  - sudo apt-get install -y ocaml-nox ocaml-native-compilers python3-setuptools python3-pip libev-dev
   - pip3 install -U Sphinx
   - ./install_local_opam2.sh
   - ./opam2_local switch create ocaml-base-compiler.4.06.0
   - eval `./opam2_local config env`
+  - ./opam2_local install conf-libev
   - ./opam2_local install -y postgresql ounit2 ocamlformat
   - make rule-check
   - psql -c 'create database links;' -U postgres

azure-pipelines.yml

@@ -67,13 +67,22 @@ steps:
   - script: |
       eval `./opam2_local env`
       ./opam2_local install ocamlformat -y
-    displayName: 'Ocamlformat'
+    displayName: 'OCamlformat'
 
   - script: |
       eval `./opam2_local env`
       make rule-check
     displayName: 'Rule check'
 
+  - bash: |
+      case $AGENT_OS in
+        "Linux") sudo apt-get install libev-dev &&
+                 eval `./opam2_local env` &&
+                ./opam2_local install conf-libev -y ;;
+        *) exit 1 ;;
+      esac
+    displayName: 'Install libev (necessary step to make Lwt 5 compile in this container)'
+
   - bash: |
       case $AGENT_OS in
         "Linux") sudo apt-get install postgresql libpq-dev sqlite3 libsqlite3-dev &&