refactor: use github actions and grcov

This commit replaces azure pipelines with github actions, tarpaulin with
(cross-platform) grcov, and codecov.io with coveralls.io.

Github Actions turned out to be a much better fit for this project. It
integrates much better with our already existing workflow, seems more
feature complete and is easier to work with.

Tarpaulin kept crashing on Azure Pipelines and after trying also with
Github Actions, its still under active development so thats not very
strange. However, I prefer to have inaccurate code coverage than no
coverage at all. Therefor I replaced Tarpaulin with grcov. grcov does
report some lines missed that are actually ok but again, its better than
nothing. Also, grcov works on all our target platforms so we get better
cross platform coverage in return.

I replaced codecov.io as our coverage service with coveralls.io.
Although I like the interface of codecov better there is no support
for uploading reports on macOS and windows. See
codecov/codecov-action#13 for more
information.

.github/actions-rs/grcov.yml

@@ -0,0 +1,8 @@
+ignore-not-existing: true
+llvm: true
+output-type: lcov
+ignore:
+  - "C:/*"
+  - "../*"
+  - "/*"
+  - "**/main.rs"

.github/actions/install-llvm/.gitattributes

@@ -0,0 +1 @@
+*.exe binary

.github/actions/install-llvm/.gitignore

@@ -0,0 +1 @@
+node_modules

.github/actions/install-llvm/action.yml

@@ -0,0 +1,5 @@
+name: Install LLVM
+description: Installs all LLVM binaries required to build Mun.
+runs:
+  using: 'node12'
+  main: 'dist/index.js'