Convert delimiters using RAII semantics (#34)

PushBrace, PushParen and PushBracket push the open delimiter in the
constructor and the closing delimiter in the destructor.

They replace the following very common pattern:

```cpp
StrCat(token::kOpenParen);
...
StrCat(token::kCloseParen);
```

with:

```cpp
PushParen push(*this);
...
```

Author: lucic71