Use escape_default() for strings in LitKind::token().

This avoids converting every char to \u{...} form, which bloats the
resulting strings unnecessarily. It also provides consistency with the
existing escape_default() calls in LitKind::token() used for raw
string literals, char literals, and raw byte char literals.

There are two benefits from this change.

- Compilation is faster. Most of the rustc-perf benchmarks see a
  non-trivial speedup, particularly for incremental rebuilds, with the
  best speedup over 13%, and multiple others over 10%.

- Generated rlibs are smaller. An extreme example is libfutures.rlib,
  which shrinks from 2073306 bytes to 1765927 bytes, a 15% reduction.

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