Convert delimiters using RAII semantics

[lucic71]

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:

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

with:

PushParen push(*this);
...