Use libsecp256k1 instead of OpenSSL for key derivation:

While most of the code associated with secp256k1 operations had
been migrated to libsecp256k1, the deterministic key derivation
code was still using calls to OpenSSL.

If merged, this commit replaces the OpenSSL-based routines with
new libsecp256k1-based implementations. No functional change is
expected and the change should be transparent.

This commit also removes several support classes and utility
functions that wrapped or adapted various OpenSSL types that
are no longer needed.

A tip of the hat to the original author of this truly superb
library, Dr. Pieter Wuille, and to all other contributors.

Builds/CMake/RippledCore.cmake

@@ -111,11 +111,8 @@ target_sources (xrpl_core PRIVATE
     main sources:
       subdir: crypto
   #]===============================]
-  src/ripple/crypto/impl/GenerateDeterministicKey.cpp
   src/ripple/crypto/impl/RFC1751.cpp
   src/ripple/crypto/impl/csprng.cpp
-  src/ripple/crypto/impl/ec_key.cpp
-  src/ripple/crypto/impl/openssl.cpp
   src/ripple/crypto/impl/secure_erase.cpp)
 
 add_library (Ripple::xrpl_core ALIAS xrpl_core)
@@ -174,16 +171,10 @@ install (
   DESTINATION include/ripple/basics)
 install (
   FILES
-    src/ripple/crypto/GenerateDeterministicKey.h
     src/ripple/crypto/RFC1751.h
     src/ripple/crypto/csprng.h
     src/ripple/crypto/secure_erase.h
   DESTINATION include/ripple/crypto)
-install (
-  FILES
-    src/ripple/crypto/impl/ec_key.h
-    src/ripple/crypto/impl/openssl.h
-  DESTINATION include/ripple/crypto/impl)
 install (
   FILES
     src/ripple/json/JsonPropertyStream.h
@@ -770,11 +761,6 @@ target_sources (rippled PRIVATE
   src/test/core/SociDB_test.cpp
   src/test/core/Stoppable_test.cpp
   src/test/core/Workers_test.cpp
-  #[===============================[
-     test sources:
-       subdir: crypto
-  #]===============================]
-  src/test/crypto/Openssl_test.cpp
   #[===============================[
      test sources:
        subdir: csf