Merge bitcoin-core/secp256k1#1000: Synthetic int128 type.

a340d9500a ci: add int128_struct tests (Jonas Nick)
dceaa1f579 int128: Tidy #includes of int128.h and int128_impl.h (Tim Ruffing)
2914bccbc0 Simulated int128 type. (Russell O'Connor)

Pull request description:

  Abstracts the int128 type and provides an native version, if available, or a implements it using a pair of int64_t's.

  This is activated by setting the configuration flag `--with-test-override-wide-multiply=int128_struct`.

  The primary purpose of this PR is to take advantage of MSVC's [umulh](https://docs.microsoft.com/en-us/cpp/intrinsics/umulh?view=msvc-170) intrinsic that we can use to simulate an int128 type which MSVC does not have (AFAIU). This PR lays out the groundwork for this level of MSVC support, but doesn't include the configuration logic to enable it yet.

  For completeness, and implementation of `umulh` and `mulh` are also provided for compilers that support neither the intrinsic nor the int128 type (such as CompCert?).  This also opens up the possibility of removing the 32-bit field and scalar implementations should that ever be desired.

ACKs for top commit:
  sipa:
    ACK a340d9500a
  jonasnick:
    ACK a340d9500a

Tree-SHA512: b4f2853fa3ab60ce9d77b4eaee1fd20c4b612850e19fcb3179d7e36986f420c6c4589ff72f0cf844f989584ace49a1cd23cca3f4e405dabefc8da647a0df679d

.cirrus.yml

@@ -68,6 +68,7 @@ task:
     - env: {WIDEMUL:  int64,  RECOVERY: yes}
     - env: {WIDEMUL:  int64,                 ECDH: yes, SCHNORRSIG: yes}
     - env: {WIDEMUL: int128}
+    - env: {WIDEMUL: int128_struct}
     - env: {WIDEMUL: int128,  RECOVERY: yes,            SCHNORRSIG: yes}
     - env: {WIDEMUL: int128,                 ECDH: yes, SCHNORRSIG: yes}
     - env: {WIDEMUL: int128,  ASM: x86_64}
@@ -271,20 +272,22 @@ task:
     EXPERIMENTAL: yes
     SCHNORRSIG: yes
     CTIMETEST: no
+    # Use a MinGW-w64 host to tell ./configure we're building for Windows.
+    # This will detect some MinGW-w64 tools but then make will need only
+    # the MSVC tools CC, AR and NM as specified below.
+    HOST: x86_64-w64-mingw32
+    CC: /opt/msvc/bin/x64/cl
+    AR: /opt/msvc/bin/x64/lib
+    NM: /opt/msvc/bin/x64/dumpbin -symbols -headers
     # Set non-essential options that affect the CLI messages here.
     # (They depend on the user's taste, so we don't want to set them automatically in configure.ac.)
     CFLAGS: -nologo -diagnostics:caret
     LDFLAGS: -XCClinker -nologo -XCClinker -diagnostics:caret
-  # Use a MinGW-w64 host to tell ./configure we're building for Windows.
-  # This will detect some MinGW-w64 tools but then make will need only
-  # the MSVC tools CC, AR and NM as specified below.
   matrix:
     - name: "x86_64 (MSVC): Windows (Debian stable, Wine)"
+    - name: "x86_64 (MSVC): Windows (Debian stable, Wine, int128_struct)"
       env:
-        HOST: x86_64-w64-mingw32
-        CC: /opt/msvc/bin/x64/cl
-        AR: /opt/msvc/bin/x64/lib
-        NM: /opt/msvc/bin/x64/dumpbin -symbols -headers
+        WIDEMUL: int128_struct
     - name: "i686 (MSVC): Windows (Debian stable, Wine)"
       env:
         HOST: i686-w64-mingw32