Add object selection (textobjects) (#385)

* Add textobjects for word

* Add textobjects for surround characters

* Apply clippy lints

* Remove ThisWordPrevBound in favor of PrevWordEnd

It's the same as PrevWordEnd except for taking the current char
into account, so use a "flag" to capture that usecase

* Add tests for PrevWordEnd movement

* Remove ThisWord* movements

They did not preserve anchor positions and were only used
for textobject boundary search anyway so replace them with
simple position finding functions

* Rewrite tests of word textobject

* Add tests for surround textobject

* Add textobject docs

* Refactor textobject word position functions

* Apply clippy lints on textobject

* Fix overflow error with textobjects

2 files changed, 20 insertions, 1 deletions

diff --git a/book/src/keymap.md b/book/src/keymap.md
index 0265fe6d..6b7ccd11 100644
--- a/book/src/keymap.md
+++ b/book/src/keymap.md
@@ -150,7 +150,8 @@ Jumps to various locations.
 ## Match mode
 
 Enter this mode using `m` from normal mode. See the relavant section
-in [Usage](./usage.md#surround) for an explanation about surround usage.
+in [Usage](./usage.md) for an explanation about [surround](./usage.md#surround)
+and [textobject](./usage.md#textobject) usage.
 
 | Key              | Description                                     |
 | -----            | -----------                                     |
@@ -158,6 +159,8 @@ in [Usage](./usage.md#surround) for an explanation about surround usage.
 | `s` `<char>`     | Surround current selection with `<char>`        |
 | `r` `<from><to>` | Replace surround character `<from>` with `<to>` |
 | `d` `<char>`     | Delete surround character `<char>`              |
+| `a` `<object>`   | Select around textobject                        |
+| `i` `<object>`   | Select inside textobject                        |
 
 ## Object mode
 
diff --git a/book/src/usage.md b/book/src/usage.md
index e6bd60e2..0458071a 100644
--- a/book/src/usage.md
+++ b/book/src/usage.md
@@ -24,3 +24,19 @@ It can also act on multiple seletions (yay!). For example, to change every occur
 - `mr([` to replace the parens with square brackets
 
 Multiple characters are currently not supported, but planned.
+
+## Textobjects
+
+Currently supported: `word`, `surround`.
+
+![textobject-demo](https://user-images.githubusercontent.com/23398472/124231131-81a4bb00-db2d-11eb-9d10-8e577ca7b177.gif)
+
+- `ma` - Select around the object (`va` in vim, `<alt-a>` in kakoune)
+- `mi` - Select inside the object (`vi` in vim, `<alt-i>` in kakoune)
+
+| Key after `mi` or `ma` | Textobject selected      |
+| ---                    | ---                      |
+| `w`                    | Word                     |
+| `(`, `[`, `'`, etc     | Specified surround pairs |
+
+Textobjects based on treesitter, like `function`, `class`, etc are planned.