Get lightbeam compiling on stable Rust (#2370)

This will hopefully remove a small thorn in our side with periodic
nightly breakage due to nightly features changing. This commit moves
lightbeam to stable Rust, swapping out `staticvec` for `arrayvec` and
otherwise updating some dependencies (namely `dynasm`) to compile with
stable.

This then also updates CI appropriately to not use a pinned nightly and
instead us a floating `nightly` channel so we can head off any breakage
coming up ASAP.

crates/lightbeam/src/disassemble.rs

@@ -2,6 +2,7 @@ use capstone::prelude::*;
 use dynasmrt::AssemblyOffset;
 use std::error::Error;
 use std::fmt::{Display, Write};
+use std::io;
 
 pub fn disassemble<D: Display>(
     mem: &[u8],
@@ -10,10 +11,11 @@ pub fn disassemble<D: Display>(
     let cs = Capstone::new()
         .x86()
         .mode(arch::x86::ArchMode::Mode64)
-        .build()?;
+        .build()
+        .map_err(map_caperr)?;
 
     println!("{} bytes:", mem.len());
-    let insns = cs.disasm_all(&mem, 0x0)?;
+    let insns = cs.disasm_all(&mem, 0x0).map_err(map_caperr)?;
     for i in insns.iter() {
         let mut line = String::new();
 
@@ -49,3 +51,7 @@ pub fn disassemble<D: Display>(
 
     Ok(())
 }
+
+fn map_caperr(err: capstone::Error) -> Box<dyn Error> {
+    Box::new(io::Error::new(io::ErrorKind::Other, err.to_string()))
+}