Add an option to not automatically pad const-generics braces with spaces

By which I mean, given this code from a crate of mine:

impl<T, const N: usize> Index<usize> for StaticVec<T, {N}> {
  type Output = T;
  ///Asserts that `index` is less than the current length of the StaticVec,
  ///and if so returns the value at that position as a constant reference.
  #[inline(always)]
  fn index(&self, index: usize) -> &Self::Output {
    assert!(index < self.length);
    unsafe { self.data.get_unchecked(index).get_ref() }
  }
}  

Rustfmt will always make it look like this:

impl<T, const N: usize> Index<usize> for StaticVec<T, { N }> {
  type Output = T;
  ///Asserts that `index` is less than the current length of the StaticVec,
  ///and if so returns the value at that position as a constant reference.
  #[inline(always)]
  fn index(&self, index: usize) -> &Self::Output {
    assert!(index < self.length);
    unsafe { self.data.get_unchecked(index).get_ref() }
  }
}  

The difference being { N } instead of the original {N}. Note that I'm not even sure if this is intentional behavior in the sense of being specific to const generics, or just something more general to do with brace spacing.

The issue names no files or tests. Start by tracing rustfmt's brace-spacing handling for const generics and reviewing existing formatting options and their tests. Done means a configuration option can preserve {N} when requested while the current default behavior remains covered.