Rename QuoteContext to Quotes

`QuoteContext` is required on all signatures. To make it simpler to keep signatures on one line we shorten the name to `Quotes`. This name will usually be used in `(using Quotes)` which reads as _using quotes_ and descibes exactly why this contextual parameter is the.

```diff
- def f[T: Type](x: Expr[T])(using QuoteContext): Expr[T] = ...
+ def f[T: Type](x: Expr[T])(using Quotes): Expr[T] = ...
```

Author: nicolasstucki