*: Partial string handling refactoring

Main points:

- Replace `char_u` with `char` in some cases.
- Remove `str[len] = NUL` hack in some cases when `str` may be considered
  `const`.

test/functional/eval/let_spec.lua

@@ -0,0 +1,22 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local clear = helpers.clear
+local meths = helpers.meths
+local redir_exec = helpers.redir_exec
+
+before_each(clear)
+
+describe(':let command', function()
+  it('correctly lists variables with curly-braces', function()
+    meths.set_var('v', {0})
+    eq('\nv                     [0]', redir_exec('let {"v"}'))
+  end)
+
+  it('correctly lists variables with subscript', function()
+    meths.set_var('v', {0})
+    eq('\nv[0]                  #0', redir_exec('let v[0]'))
+    eq('\ng:["v"][0]            #0', redir_exec('let g:["v"][0]'))
+    eq('\n{"g:"}["v"][0]        #0', redir_exec('let {"g:"}["v"][0]'))
+  end)
+end)