Convert theme_select() to Twig

Converted theme_select to twig.

Patch attached.

2: drupal-convert_theme_select_to_twig-2152225-2.patch queued for re-testing.

Here is a manual test of http://d8.dev/admin/config/people/accounts/display
There is no difference in the markup except whitespace.

Some nitpicks to take care of.

1. +++ b/core/includes/form.inc
   @@ -812,7 +812,9 @@ function form_process_select($element) {
   + * Preprocesses variables for theme_select().
   

   This should be Prepares variables for select element templates.

2. +++ b/core/includes/form.inc
   @@ -812,7 +812,9 @@ function form_process_select($element) {
   + * Default template: select.html.twig
   

   Needs a period.

3. +++ b/core/modules/system/templates/select.html.twig
   @@ -0,0 +1,15 @@
   + * Available variables
   

   Needs a colon ':' at the end of this line.

4. +++ b/core/modules/system/templates/select.html.twig
   @@ -0,0 +1,15 @@
   + * - options: HTML string of options.
   

   Don't write the variable types. This should be something like "The option element children." or something along those lines. https://drupal.org/node/1823416#datatypes

Thanks @longwave! This is ready for profiling and then RTBC.

Scenario:
admin/structure/types/manage/article/form-display
with a datetime field too.

=== 8.x..8.x compared (530ef14e56c65..530ef19752730):

ct  : 139,156|139,156|0|0.0%
wt  : 783,110|773,268|-9,842|-1.3%
cpu : 748,393|739,540|-8,853|-1.2%
mu  : 36,372,936|36,372,936|0|0.0%
pmu : 36,487,352|36,487,352|0|0.0%

http://www.lionsad.de/xhprof-kit/xhprof/xhprof_html/?run1=530ef14e56c65&...

=== 8.x..2152225-twig-theme_select compared (530ef14e56c65..530ef2275628d):

ct  : 139,156|139,675|519|0.4%
wt  : 783,110|776,100|-7,010|-0.9%
cpu : 748,393|741,035|-7,358|-1.0%
mu  : 36,372,936|36,401,200|28,264|0.1%
pmu : 36,487,352|36,516,040|28,688|0.1%

http://www.lionsad.de/xhprof-kit/xhprof/xhprof_html/?run1=530ef14e56c65&...

Committed and pushed to 8.x. Thanks!