Update secp256k1 to 0.3.2 (#4653)

Copy the new code to `src/secp256k1` without changes:
`src/secp256k1` is identical to bitcoin-core/secp256k1@acf5c55 (v0.3.2).

We could consider changing to a Git submodule, though that would require
changes to the build instructions because we are not using submodules
anywhere else.

Author: thejohnfreeman

CMakeLists.txt

@@ -71,7 +71,9 @@ find_package(OpenSSL 1.1.1 REQUIRED)
 set_target_properties(OpenSSL::SSL PROPERTIES
   INTERFACE_COMPILE_DEFINITIONS OPENSSL_NO_SSL2
 )
+set(SECP256K1_INSTALL TRUE)
 add_subdirectory(src/secp256k1)
+add_library(secp256k1::secp256k1 ALIAS secp256k1)
 add_subdirectory(src/ed25519-donna)
 find_package(lz4 REQUIRED)
 # Target names with :: are not allowed in a generator expression.