feat: new fiber abstractions

There are now 2 types of fibers
* Immediate - a fiber that starts executing immediately
* Deferred  - a fiber that is scheduled for execution
Both of them come in 2 variants, consider the case of Immediate:
* Immediate<F, T>  - has a fiber function that returns a T
* UnitImmediate<F> - has a fiber function that returns nothing. This is
                     an optimized version of Immediate<F, ()>.
Same with Deferred and UnitDeferred.

All of these fibers are joinable. Non-joinable fibers are not supported
yet.

Author: gmoshkin

src/error.rs

@@ -27,6 +27,9 @@ use rmp::encode::ValueWriteError;
 
 use crate::ffi::tarantool as ffi;
 
+/// A specialized [`Result`] type for the crate
+pub type Result<T> = std::result::Result<T, Error>;
+
 /// Represents all error cases for all routines of crate (including Tarantool errors)
 #[derive(Debug, Fail)]
 pub enum Error {
@@ -171,7 +174,7 @@ pub struct TarantoolError {
 impl TarantoolError {
     /// Tries to get the information about the last API call error. If error was not set
     /// returns `Ok(())`
-    pub fn maybe_last() -> Result<(), Self> {
+    pub fn maybe_last() -> std::result::Result<(), Self> {
         let error_ptr = unsafe { ffi::box_error_last() };
         if error_ptr.is_null() {
             return Ok(());

src/ffi/helper.rs

@@ -3,3 +3,19 @@ use std::ffi::CString;
 pub unsafe fn new_c_str(s: &str) -> CString {
     return CString::new(s).unwrap();
 }
+
+#[macro_export]
+macro_rules! c_str {
+    ($s:literal) => {
+        ::std::ffi::CStr::from_bytes_with_nul_unchecked(
+            ::std::concat!($s, "\0").as_bytes()
+        )
+    };
+}
+
+#[macro_export]
+macro_rules! c_ptr {
+    ($s:literal) => {
+        crate::c_str!($s).as_ptr()
+    };
+}