v/checker: Warn if unsafe method called outside unsafe block

[ntrel]

Also mark ustring.free as unsafe.
Unsafe functions are not checked yet.

[ntrel]

I assume the windows-tcc test failure is unrelated:

FAIL  [ 55/220]   280.876 ms vlib\os\inode_test.v
testsuite_begin, tfolder = C:\Users\RUNNER~1\AppData\Local\Temp\v\tests\inode_test

D:\\a\\v\\v\\vlib\\os\\inode_test.v:16: failed assert in function testsuite_begin
Source  : `main.os.is_dir(main.tfolder)`