no_std support (#232)

- Disable default features in dependencies
- Use anyhow::Error::msg to map errors. Note: anyhow will itself be removed later.
- lazy_static/spin_no_std used in no_std environments
- ensure_no_std binary is built to target  thumbv7m-none-eabi to ensure that
  there are no std dependencies.  thumbv7m-none-eabi target has no std support.
- The opa-no-std feature enables only those Regorus features that work with no_std.
- Enable tests with no_std
- Update sizes of regorus binary in  README.md
- Ensure that regorus example can be built with only std
- Ensure that regorus example can be built with no_std

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/arc.rs

@@ -1,11 +1,13 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+#![allow(unused)]
 use lazy_static::lazy_static;
 use std::sync::Mutex;
 
 use regorus::*;
 
+#[cfg(feature = "arc")]
 // Ensure that types can be s
 lazy_static! {
     static ref VALUE: Value = Value::Null;
@@ -14,6 +16,7 @@ lazy_static! {
 }
 
 #[test]
+#[cfg(feature = "arc")]
 fn shared_engine() -> anyhow::Result<()> {
     let e_guard = ENGINE.lock();
     let mut engine = e_guard.expect("failed to lock engine");

tests/ensure_no_std/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "ensure_no_std"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+anyhow = { version = "1.0.83", default-features = false }
+regorus = { path = "../..", default-features = false, features = ["opa-no-std"] }

tests/ensure_no_std/src/main.rs

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+#![no_std]
+#![no_main]
+
+use core::panic::PanicInfo;
+
+#[panic_handler]
+fn panic(_info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[no_mangle]
+pub extern "C" fn _start() -> ! {
+    loop {}
+}

tests/lexer/mod.rs

@@ -225,6 +225,7 @@ fn invalid_line() -> Result<()> {
 }
 
 #[test]
+#[cfg(feature = "std")]
 fn file_more_than_64_kb_size() -> Result<()> {
     let source = Source::from_file("tests/kata/data/large.rego")?;
     let mut lexer = Lexer::new(&source);

tests/mod.rs

@@ -8,6 +8,3 @@ mod engine;
 mod lexer;
 mod parser;
 mod value;
-
-#[cfg(feature = "arc")]
-mod arc;