Use built-in expand() without check when fern.vim is not loaded.

[gw31415]

Thank you for the wonderful plugin.

The current implementation uses fern#util#expand in s:expand even on the first BufEnter.
This causes it to load even on the first startup even if it is not necessary to load fern.vim, for example when opening a file (not a directory).

This PR changes s:expand to use the built-in expand() function without checking when fern.vim is not loaded.
This enables for lazy loading of fern.vim, which should improve startup speed.

Thanks.

[lambdalisue]

I see. Well, then let's check the presence of fern#util#expand itself instead and describe the reason as a function comment (outside of the function) like

" explain why we check the presence of `fern#util#expand` which usually shoud not
function! s:expand(expr) abort
  if exists('fern#util#expand')
    return fern#util#expand(a:expr)
  endif
  return expand(a:expr)
endfunction

[gw31415]

Thanks for the advice on better changes. I've made the changes.

[gw31415]

Thanks!