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/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+edition = "2021"
+name = "libhackrf"
+version = "0.1.1"
+authors = ["Connor Slade <connor@connorcode.com>"]
+description = "A modern libhackrf wrapper that supports receiving and transmitting. (Patched for ARM cross-compilation)"
+license = "MIT"
+repository = "https://github.com/connorslade/libhackrf-rs"
+
+[lib]
+name = "libhackrf"
+path = "src/lib.rs"
+doctest = false
+
+[dependencies.num-complex]
+version = "0.4.6"