perf(loader): use a quicker version of vim.fs.normalize

Problem:

vim.fs.normalize() normalizes too much vim.loader and is slow.

Solution:

Make it faster by doing less. This reduces the times spent in
vim.fs.normalize in vim.loader from ~13ms -> 1-2ms.

Numbers from a relative benchmark:
- Skipping `vim.validate()`: 285ms -> 230ms
- Skipping `path_resolve_dot()`: 285ms -> 60ms
- Skipping `double_slash`: 60ms -> 35ms

runtime/lua/vim/loader.lua

@@ -85,7 +85,7 @@ function Loader.get_hash(path)
 end
 
 local function normalize(path)
-  return fs.normalize(path, { expand_env = false })
+  return fs.normalize(path, { expand_env = false, _fast = true })
 end
 
 --- Gets the rtp excluding after directories.