Update assembler infrastructure to be more modular

Assembler has more discrete passes now, but things are kind of bare on
the error message front. But, everything is working as it did
previously, so we can move on to more interesting things.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/main.rs

@@ -107,13 +107,11 @@ fn get_writer(path: impl AsRef<Path>) -> Result<Box<dyn Write>> {
 fn main() -> Result<()> {
     use vm::{
         state::State,
-        obj::assemble::AsmSession,
+        obj::assemble,
     };
 
     let opt = Options::from_args();
-    let mut asm_session = AsmSession::default();
-    asm_session.include(&opt.input)?;
-    let object = asm_session.assemble()?;
+    let object = assemble::assemble_path(&opt.input)?;
 
     if opt.compile_only {
         let outfile = opt.out.clone().unwrap_or_else(|| {