Rename `text` to `lettering`

The function `text :: Text -> Picture` is confusing, because its name suggests that it's describing text, but it's actually describing a picture made from text. I propose we use the word `writing` instead. This is a word whose meaning actually suggests the visual appearance of letters.

Other noun options incluse `print`, `lettering`, or `label`; but they have various issues of their own. There are some adjective options, too, like `written`; but all other primitive picture functions are already named with nouns - and `printed` is already used for the conversion from Number to Text, so that would be confusing.

FYI @alphalambda

Haskell の定義と `text :: Text -> Picture` のすべての使用箇所を特定します。タイトルと提案が一致していないため、まず承認されている置換名を決定してください。このプリミティブが公開および参照されているすべての箇所で、選択した名詞が一貫して適用されていれば完了です。

索引モデルが issue の本文から書いたものです。