Fix ARM compilation: vendor libhackrf with c_char portability fix

The upstream libhackrf v0.1.1 crate fails to compile on ARM because
c_char is u8 on ARM (not i8 like on x86). The transmute at lib.rs:102
explicitly casts &[i8] -> &[u8], but on ARM the source is already &[u8],
causing a type mismatch.

Vendored the crate at vendor/libhackrf/ with the transmute replaced by
slice::from_raw_parts with a pointer cast, which works on both
architectures. Also cleaned up transmute calls in transfer.rs.

Fixes #1

vendor/libhackrf/src/util.rs

@@ -0,0 +1,21 @@
+use num_complex::Complex;
+
+pub trait ToComplexI8 {
+    fn to_i8(self) -> Complex<i8>;
+}
+
+pub trait ToComplexF32 {
+    fn to_f32(self) -> Complex<f32>;
+}
+
+impl ToComplexI8 for Complex<f32> {
+    fn to_i8(self) -> Complex<i8> {
+        Complex::new((self.re * 127.0) as i8, (self.im * 127.0) as i8)
+    }
+}
+
+impl ToComplexF32 for Complex<i8> {
+    fn to_f32(self) -> Complex<f32> {
+        Complex::new(self.re as f32 / 127.0, self.im as f32 / 127.0)
+    }
+}