SRFI 15 (fluid-let)

AFAICT Chez Scheme's `fluid-let` is compatible with [SRFI 15](https://srfi.schemers.org/srfi-15/srfi-15.html). In that case, this would be a compliant implementation of the latter:

```Scheme
(library (srfi :15)
  (export fluid-let)
  (import (only (chezscheme) fluid-let)))
```

The SRFI also supplied a portable implementation using `dynamic-wind`.