Remove useless libc, elf and ucontext, add comments to exceptions

improve propagation of errors in raise_exception

Cargo.lock

@@ -7,7 +7,6 @@ name = "burritos"
 version = "0.1.0"
 dependencies = [
  "cc",
- "libc",
 ]
 
 [[package]]
@@ -15,9 +14,3 @@ name = "cc"
 version = "1.0.79"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "50d30906286121d95be3d479533b458f87493b30a4b5f79a607db8f5d11aa91f"
-
-[[package]]
-name = "libc"
-version = "0.2.140"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "99227334921fae1a979cf0bfdfcc6b3e5ce376ef57e16fb6fb3ea2ed6095f80c"