Replaced old CI with one using GitHub Actions (#4)

The old CI was meant for cortex-m-semihosting, so it would not have
worked with this repository, and it also used Travis CI, whereas
the new one uses GitHub Actions.

The CI runs `cargo check` for the following targets:
- `riscv32i-unknown-none-elf`
- `riscv32imc-unknown-none-elf`
- `riscv32imac-unknown-none-elf`
- `riscv64imac-unknown-none-elf`
- `riscv64gc-unknown-none-elf`

It also checks formatting and doc.

Author: FawazTirmizi

.github/workflows/check.yml

@@ -0,0 +1,30 @@
+name: check
+on: [push, pull_request]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        # All generated code should be running on stable now, MRSV is 1.59.0
+        toolchain: [nightly, stable, 1.59.0]
+        target: [riscv32i-unknown-none-elf, riscv32imc-unknown-none-elf, riscv32imac-unknown-none-elf, riscv64imac-unknown-none-elf, riscv64gc-unknown-none-elf]
+
+        include:
+          # Nightly is only for reference and allowed to fail
+          - rust: nightly
+            experimental: true
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ matrix.toolchain }}
+          target: ${{ matrix.target }}
+          override: true
+      - uses: actions-rs/cargo@v1
+        with:
+          use-cross: true
+          command: check
+          args: --verbose --target ${{ matrix.target }}

.github/workflows/doc.yml

@@ -0,0 +1,17 @@
+name: doc
+on: [push, pull_request]
+
+
+jobs:
+  doc:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - uses: actions-rs/cargo@v1
+        with:
+          command: doc

.github/workflows/fmt.yml

@@ -0,0 +1,18 @@
+name: fmt
+on: [push, pull_request]
+
+jobs:
+  fmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+          components: rustfmt
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all -- --check

.travis.yml

@@ -20,6 +20,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
       bumped to 1.59.0
 - Clean up documentation, removing unnecessary references to
   cortex-m-semihosting and improving clarity.
+- Added GitHub Actions CI
 
 ## [v0.0.1] - 2018-02-27