fix: varint_read_u64 panics on unwrap in no_std environment (#145)

The goal of this pull request is to refactor `unwrap()` to return the `u64` value or an`Error:VarIntDecodeError` in the `no_std` environment.

This error was discovered in fuzz testing.

Tests were refactored to include a new `no_std_tests` module, in order to test the fix. 
`cargo test --no-default-features` will execute the `no_std` test.

README.md

@@ -63,6 +63,8 @@ You can also run this example from this checkout with `cargo run --example readm
 
 You can run the tests using this command: `cargo test --all-features`
 
+You can run the tests for `no_std` using this command: `cargo test --no-default-features`
+
 ## Maintainers
 
 Captain: [@dignifiedquire](https://github.com/dignifiedquire).

examples/readme.rs

@@ -1,10 +1,9 @@
-use cid::Cid;
-use multihash_codetable::{Code, MultihashDigest};
-use std::convert::TryFrom;
-
-const RAW: u64 = 0x55;
-
+#[cfg(feature = "std")]
 fn main() {
+    use cid::Cid;
+    use multihash_codetable::{Code, MultihashDigest};
+
+    const RAW: u64 = 0x55;
     let h = Code::Sha2_256.digest(b"beep boop");
 
     let cid = Cid::new_v1(RAW, h);
@@ -21,3 +20,8 @@ fn main() {
     );
     println!("{}", cid_string);
 }
+#[cfg(not(feature = "std"))]
+// cargo test --no-default-features complains without this
+fn main() {
+    print!("no_std");
+}

src/cid.rs

@@ -38,7 +38,10 @@ pub(crate) fn varint_read_u64<R: io::Read>(mut r: R) -> Result<u64> {
         if n == 0 {
             return Err(Error::VarIntDecodeError);
         } else if decode::is_last(b[i]) {
-            return Ok(decode::u64(&b[..=i]).unwrap().0);
+            match decode::u64(&b[..=i]) {
+                Ok((value, _)) => return Ok(value),
+                Err(_) => return Err(Error::VarIntDecodeError),
+            }
         }
     }
     Err(Error::VarIntDecodeError)