libsecp256k1: add runtime support for 0.3.x

this replaces https://github.com/spesmilo/electrum/pull/8320

see f6bef03c0a/CHANGELOG.md

I am not yet sure how this will look like going forward, but unless there will
be lots of libsecp256k1 releases with ~invisible harmless ABI changes, I think
conceptually this is the right approach.

electrum/ecc_fast.py

@@ -37,16 +37,23 @@ class LibModuleMissing(Exception): pass
 
 
 def load_library():
+    # note: for a mapping between bitcoin-core/secp256k1 git tags and .so.V libtool version numbers,
+    #       see https://github.com/bitcoin-core/secp256k1/pull/1055#issuecomment-1227505189
+    tested_libversions = [2, 1, 0, ]  # try latest version first
+    libnames = []
     if sys.platform == 'darwin':
-        libnames = ['libsecp256k1.1.dylib', 'libsecp256k1.0.dylib', ]
+        for v in tested_libversions:
+            libnames.append(f"libsecp256k1.{v}.dylib")
     elif sys.platform in ('windows', 'win32'):
-        libnames = ['libsecp256k1-1.dll', 'libsecp256k1-0.dll', ]
+        for v in tested_libversions:
+            libnames.append(f"libsecp256k1-{v}.dll")
     elif 'ANDROID_DATA' in os.environ:
-        libnames = ['libsecp256k1.so', ]
-    elif 'freebsd' in sys.platform:
-        libnames = ['libsecp256k1.so', ]
+        libnames = ['libsecp256k1.so', ]  # don't care about version number. we built w/e is available.
     else:  # desktop Linux and similar
-        libnames = ['libsecp256k1.so.1', 'libsecp256k1.so.0', ]
+        for v in tested_libversions:
+            libnames.append(f"libsecp256k1.so.{v}")
+        # maybe we could fall back to trying "any" version? maybe guarded with an env var?
+        #libnames.append(f"libsecp256k1.so")
     library_paths = []
     for libname in libnames:  # try local files in repo dir first
         library_paths.append(os.path.join(os.path.dirname(__file__), libname))