Reduce unwraps

Remove a number of unwraps. In some cases, a `?` just worked as is. I also made it possible to use `?` to go from Result<OutputStream, ShellError> to OutputStream. Finally, started updating PerItemCommand to be able to use the signature deserialization logic, which substantially reduces unwraps.

This is still in-progress work, but tests pass and it should be clear to merge and keep iterating on master.

src/shell/filesystem_shell.rs

@@ -7,12 +7,19 @@ use crate::shell::shell::Shell;
 use rustyline::completion::FilenameCompleter;
 use rustyline::hint::{Hinter, HistoryHinter};
 use std::path::{Path, PathBuf};
+
 pub struct FilesystemShell {
     crate path: String,
     completer: NuCompleter,
     hinter: HistoryHinter,
 }
 
+impl std::fmt::Debug for FilesystemShell {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "FilesystemShell @ {}", self.path)
+    }
+}
+
 impl Clone for FilesystemShell {
     fn clone(&self) -> Self {
         FilesystemShell {

src/shell/shell.rs

@@ -3,7 +3,7 @@ use crate::context::SourceMap;
 use crate::errors::ShellError;
 use crate::stream::OutputStream;
 
-pub trait Shell {
+pub trait Shell: std::fmt::Debug {
     fn name(&self, source_map: &SourceMap) -> String;
     fn ls(&self, args: EvaluatedWholeStreamCommandArgs) -> Result<OutputStream, ShellError>;
     fn cd(&self, args: EvaluatedWholeStreamCommandArgs) -> Result<OutputStream, ShellError>;

src/shell/shell_manager.rs

@@ -7,7 +7,7 @@ use crate::stream::OutputStream;
 use std::error::Error;
 use std::sync::{Arc, Mutex};
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct ShellManager {
     crate shells: Arc<Mutex<Vec<Box<dyn Shell + Send>>>>,
 }

src/shell/value_shell.rs

@@ -5,7 +5,7 @@ use crate::shell::shell::Shell;
 use std::ffi::OsStr;
 use std::path::PathBuf;
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct ValueShell {
     crate path: String,
     crate value: Tagged<Value>,