patch 8.1.1554: docs and tests for :const can be improved

Problem:    Docs and tests for :const can be improved.
Solution:   Improve documentation, add a few more tests. (Ryuichi Hayashida,
            closes #4551)
diff --git a/src/testdir/test_const.vim b/src/testdir/test_const.vim
index bb47e5e..d4dccb1 100644
--- a/src/testdir/test_const.vim
+++ b/src/testdir/test_const.vim
@@ -21,6 +21,21 @@
     hello
     EOS
 
+    call assert_true(exists('i'))
+    call assert_true(exists('f'))
+    call assert_true(exists('s'))
+    call assert_true(exists('F'))
+    call assert_true(exists('l'))
+    call assert_true(exists('d'))
+    if has('channel')
+        call assert_true(exists('j'))
+        call assert_true(exists('c'))
+    endif
+    call assert_true(exists('b'))
+    call assert_true(exists('n'))
+    call assert_true(exists('bl'))
+    call assert_true(exists('here'))
+
     call assert_fails('let i = 1', 'E741:')
     call assert_fails('let f = 1.1', 'E741:')
     call assert_fails('let s = "vim"', 'E741:')
@@ -218,6 +233,17 @@
     call assert_fails('const &g:encoding = "utf-8"', 'E996:')
 endfunc
 
+func Test_const_with_eval_name()
+    let s = 'foo'
+
+    " eval name with :const should work
+    const abc_{s} = 1
+    const {s}{s} = 1
+
+    let s2 = 'abc_foo'
+    call assert_fails('const {s2} = "bar"', 'E995:')
+endfunc
+
 func Test_lock_depth_is_1()
     const l = [1, 2, 3]
     const d = {'foo': 10}
diff --git a/src/version.c b/src/version.c
index aa11b7e..fcdf562 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1554,
+/**/
     1553,
 /**/
     1552,