DrNim (Nim compiler with Z3 integration) (#13743)

* code cleanups and feature additions
* added basic test and koch/CI integration
* make it build on Unix
* DrNim: now buildable on Unix, only takes 10 minutes, enjoy
* added basic documentation for DrNim which can also be seen as the RFC we're following
* drnim: change the build setup so that drnim.exe ends up in bin/
* makes simple floating point ranges work
* added basic float range check
* drnim: teach Z3 about Nim's range types plus code refactoring
* drnim: make unsigned numbers work
* added and fixed index checking under setLen
* first implementation of .ensures, .invariant and .assume (.requires still missing and so is proc type compatibility checking
* drnim: .requires checking implemented
* drnim: implemented .ensures properly
* more impressive test involving min()
* drnim: check for proc type compatibility and base method compatibility wrt .requires and .ensures
* testament: support for 'pattern <directory>
* koch: uses new <directory> feature of testament
* drnim: added tiny musings about 'old'
* Make testament work with old SSL versions
* koch: add support for 'koch drnim -d:release'
* drnim: preparations for the param.old notation

drnim/tests/tsetlen_invalidates.nim

@@ -0,0 +1,22 @@
+discard """
+  nimout: '''
+tsetlen_invalidates.nim(15, 12) Warning: cannot prove: 0 <= len(a) + -1; counter example: a.len -> 0 [IndexCheck]
+'''
+  cmd: "drnim $file"
+  action: "compile"
+"""
+
+{.push staticBoundChecks: defined(nimDrNim).}
+
+proc p() =
+  var a = newSeq[int](3)
+  if a.len > 0:
+    a.setLen 0
+    echo a[0]
+
+  if a.len > 0:
+    echo a[0]
+
+{.pop.}
+
+p()