ltm: update to 1.2.0.

Partially based on diff sent by Kai-Uwe Eckhardt in private mail.

v1.2.0
      -- A huge refactoring of the library happened - renaming,
          deprecating and replacing existing functions by improved API's.

          All deprecated functions, macros and symbols are only marked as such
          so this version is still API and ABI compatible to v1.x.

      -- Daniel Mendler was pushing for those changes and contributing a load of patches,
          refactorings, code reviews and whatnotelse.
      -- Christoph Zurnieden re-worked internals of the library, improved the performance,
          did code reviews and wrote documentation.
      -- Francois Perrad did some refactoring and took again care of linting the sources and
          provided all fixes.
      -- Jan Nijtmans, Karel Miko and Joachim Breitner contributed various patches.

      -- Private symbols can now be hidden for the shared library builds, disabled by default.
      -- All API's follow a single code style, are prefixed the same etc.
      -- Unified, safer and improved API's
      -- Less magic numbers - return values (where appropriate) and most flags are now enums,
          this was implemented in a backwards compatible way where return values were int.
      -- API's with return values are now by default marked as "warn on unsused result", this
          can be disabled if required (which will most likely hide bugs), c.f. MP_WUR in tommath.h
      -- Provide a whole set of setters&getters for different primitive types (long, uint32_t, etc.)
      -- All those primitive setters are now optimized.
      -- It's possible to automatically tune the cutoff values for Karatsuba&Toom-Cook
      -- The custom allocators which were formerly known as XMALLOC(), XFREE() etc. are now available
          as MP_MALLOC(), MP_REALLOC(), MP_CALLOC() and MP_FREE(). MP_REALLOC() and MP_FREE() now also
          provide the allocated size to ease the usage of simple allocators without tracking.
      -- Building is now also possible with MSVC 2015, 2017 and 2019 (use makefile.msvc)
      -- Added mp_decr() and mp_incr()
      -- Added mp_log_u32()
      -- Improved prime-checking
      -- Improved Toom-Cook multiplication
      -- Removed the LTM book (`make docs` now builds the user manual)

(wiz)