cli: Add diff parsing

Parse Git's unified diff format into Radicle Surf's types.  Support to
and from the format helps testing and debugging.

Split the UnifiedDiff trait into Decode and Encode traits so we
can offer Decode in cases where no data could be expected..
For example Option<Hunk<_>> and Hunk<_> allowing partial and repeated
decoding on a byte stream.

Avoid decoding `Diff`, `DiffContent::Binary`, `FileDiff`, and `FileHeader`
types.  `Diff` keeps fields and methods private which are necessary for
decoding.

radicle-cli/tests/data/diff.diff

@@ -0,0 +1,34 @@
+diff --git a/README.md b/README.md
+index 18fb832..ce60f40 100644
+--- a/README.md
++++ b/README.md
+@@ -4,8 +4,8 @@ componentwise
+ reusing
+ simplest
+ crag
+-offended
+ omitting
++offended
+ grinders
+ cutting
+ proverb
+@@ -21,8 +21,8 @@ alliterative
+ stays
+ mahogany
+ whole
+-snowflake
+ ellipses
++snowflake
+ yearning
+ inciting
+ permeating
+@@ -30,3 +30,9 @@ bereaved
+ eventfully
+ hears
+ vacationing
++
++imprisoning
++absinthe
++shack
++exhale
++perfected