Fix Vim float highlighting and some keyword highlight

### Fix up float highlighting in Vim.

This fixes a regression introduced in #10793.

Having a colorscheme which highlights Float the same as Number (I
believe most do), I hadn't noticed that having the special case of "5."
floats (which was one of the added features in #10793) last made it take
precedence, and so it was left to @thestinger to notice it.

The regression meant that in `5.0`, the `5.` was a `rustFloat` (linked
by default to `Float`) and the `0` was a `rustDecNumber` (linked by
default to `Number`), and for `5.0f32` the `5.` was a `rustFloat` and
the `0f32` was a second `rustFloat` (and thus appeared correctly, though
for the wrong reason).

### Vim keyword highlighting improvements.

- Removed the `log` keyword;
- Removed keyword duplicates;
- Highlighted `const` as `Error` rather than `StorageClass`; and
- Highlighted all the reserved keywords as `Error` rather than as
  `Keyword`.

(As usual, these highlightings can be overridden if desired.)