eval: Move get_tv_lnum and get_tv_float to eval/typval.h

Additionally

- Rename former tv_get_float to tv_get_float_chk due to name conflict (former
  get_tv_float is better suited for being tv_get_float).
- Add E907 error to get_tv_float() and test that it is being raised when
  appropriate.

test/functional/eval/sort_spec.lua

@@ -0,0 +1,41 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local NIL = helpers.NIL
+local eval = helpers.eval
+local clear = helpers.clear
+local meths = helpers.meths
+local funcs = helpers.funcs
+local command = helpers.command
+local exc_exec = helpers.exc_exec
+
+before_each(clear)
+
+describe('sort()', function()
+  it('errors out when sorting special values', function()
+    eq('Vim(call):E907: Using a special value as a Float',
+       exc_exec('call sort([v:true, v:false], "f")'))
+  end)
+
+  it('sorts “wrong” values between -0.0001 and 0.0001, preserving order',
+  function()
+    meths.set_var('list', {true, false, NIL, {}, {a=42}, 'check',
+                           0.0001, -0.0001})
+    command('call insert(g:list, function("tr"))')
+    local error_lines = funcs.split(
+        funcs.execute('silent! call sort(g:list, "f")'), '\n')
+    local errors = {}
+    for _, err in ipairs(error_lines) do
+      errors[err] = true
+    end
+    eq({
+      ['E891: Using a Funcref as a Float']=true,
+      ['E892: Using a String as a Float']=true,
+      ['E893: Using a List as a Float']=true,
+      ['E894: Using a Dictionary as a Float']=true,
+      ['E907: Using a special value as a Float']=true,
+    }, errors)
+    eq('[-1.0e-4, function(\'tr\'), v:true, v:false, v:null, [], {\'a\': 42}, \'check\', 1.0e-4]',
+       eval('string(g:list)'))
+  end)
+end)