FFI: Create error handling code and LAST_ERROR static

Any errors reported will cause the C API functions to return an error
value (NULL or -1). The caller can then use:

* const char* tts_get_error() to get a pointer to a string describing
  the error
* void tts_clear_error() to deallocate any currently stored error
message.

src/ffi.rs

@@ -0,0 +1,37 @@
+use libc::c_char;
+use std::{
+    cell::RefCell,
+    ffi::{CString, NulError},
+    ptr,
+};
+
+thread_local! {
+    /// Stores the last reported error, so it can be retrieved at will from C
+    static LAST_ERROR: RefCell<Option<CString>> = RefCell::new(None);
+}
+
+fn set_last_error<E: Into<Vec<u8>>>(err: E) -> Result<(), NulError> {
+    LAST_ERROR.with(|last| {
+        *last.borrow_mut() = Some(CString::new(err)?);
+        Ok(())
+    })
+}
+
+/// Get the last reported error as a const C string (const char*)
+/// This string will be valid until at least the next call to `tts_get_error`.
+/// It is never called internally by the library.
+#[no_mangle]
+pub extern "C" fn tts_get_error() -> *const c_char {
+    LAST_ERROR.with(|err| match &*err.borrow() {
+        Some(e) => e.as_ptr(),
+        None => ptr::null(),
+    })
+}
+
+/// Deallocate the last reported error (if any).
+#[no_mangle]
+pub extern "C" fn tts_clear_error() {
+    LAST_ERROR.with(|err| {
+        *err.borrow_mut() = None;
+    });
+}