Refactor

This refactors a huge chunk of the code base to make it more maintainable.

Main points:

* Proper separation between bin and lib. Bin handles argument parsing &
  validation and (most of) the output. Lib provides interfaces for all
  opreations.

* Before, libgit2 internals were literred throughout the codebase,
  mainly the `Repository` struct and `git2::Error` in Results. They
  library is now properly wrapped in `repo.rs`, which exposes only the
  required functionality. It also standardizes the Error messages
  (they're just Strings for now) and handles stuff like the copious
  usage of Options to wrap maybe-invalid-utf-8 values. The program will
  still panic on non-utf-8 Strings e.g. in git remotes, but I guess this
  is acceptable. If you actually manage to hit this case, I promise I'll
  fix it :D

* Many unwraps() are now gone and properly handled.

* The table printing functionality is now confined to `table.rs`,
  instead of passing tables as parameters through the whole program.

src/config.rs

@@ -1,18 +1,48 @@
 use serde::{Deserialize, Serialize};
 
-use super::repo::Repo;
+use super::repo::RepoConfig;
 
 #[derive(Debug, Serialize, Deserialize)]
 #[serde(deny_unknown_fields)]
 pub struct Config {
-    pub trees: Vec<Tree>,
+    pub trees: Trees,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Trees(Vec<Tree>);
+
+impl Trees {
+    pub fn to_config(self) -> Config {
+        Config { trees: self }
+    }
+
+    pub fn from_vec(vec: Vec<Tree>) -> Self {
+        Trees(vec)
+    }
+
+    pub fn as_vec(self) -> Vec<Tree> {
+        self.0
+    }
+
+    pub fn as_vec_ref(&self) -> &Vec<Tree> {
+        self.0.as_ref()
+    }
+}
+
+impl Config {
+    pub fn as_toml(&self) -> Result<String, String> {
+        match toml::to_string(self) {
+            Ok(toml) => Ok(toml),
+            Err(error) => Err(error.to_string()),
+        }
+    }
 }
 
 #[derive(Debug, Serialize, Deserialize)]
 #[serde(deny_unknown_fields)]
 pub struct Tree {
     pub root: String,
-    pub repos: Option<Vec<Repo>>,
+    pub repos: Option<Vec<RepoConfig>>,
 }
 
 pub fn read_config(path: &str) -> Result<Config, String> {