Revive FlexTLS

[sishtiaq]

@beurdouche @karthikbhargavan @sishtiaq

Requirement: We need a flextls.fst, to exercise the internal APIs directly, initially towards better testing of the record

Library status: The source code has been copied over from my F# code. It has been re-modularized in a slightly different way and prefixed with the FlexTLS namespace.

I expect most of the work for the porting to be in two phases, syntactic issues related to F* and changing the function calls to the new internal APIs of miTLS*

Goals: • I want to have a lax type checking and extracted version as soon as spare time allows • I don't believe that any verification is required as anyway it's goal is to break abstractions • Improve the logging process and the success/failure descriptions Improve heuristics for trace generation towards a much better SmackTLS

[sishtiaq]

Depends on #39 being done.

[vkanne-msft]

@sishtiaq . pulling it in as a placeholder to see if this is something we can get started with in M1.