Update Bitcoin Core dependency to 0.21.2

Currently we are vendoring the source code from Bitcoin Core `v0.20.1`.

Update the subtree to use branch `0.21.2` using:

git subtree pull --prefix='depend/bitcoin' \
                 git@github.com:bitcoin/bitcoin.git v0.21.2 \
                 --squash

Includes changes to the build script by Jake and update to the C++
toolchain suggested by Richard.

Uses the new version format, increasing the Bitocoin Core version as is
applicable and also bumping the minor version of this lib.

Co-developed-by: Jake Rawsthorne <jake@jakerawsthorne.co.uk>
Co-developed-by: Richard Ulrich <richard.ulrich@seba.swiss>

build.rs

@@ -21,15 +21,21 @@ fn main() {
     let is_big_endian = env::var("CARGO_CFG_TARGET_ENDIAN").expect("No endian is set") == "big";
     let mut base_config = cc::Build::new();
     base_config
-        .cpp(true)
-        .include("depend/bitcoin/src")
         .include("depend/bitcoin/src/secp256k1/include")
         .define("__STDC_FORMAT_MACROS", None);
 
+    if target.contains("windows") {
+        base_config.define("WIN32", "1");
+    }
+
+    let mut secp_config = base_config.clone();
+    let mut consensus_config = base_config;
+
     // **Secp256k1**
     if !cfg!(feature = "external-secp") {
-        base_config
+        secp_config
             .include("depend/bitcoin/src/secp256k1")
+            .include("depend/bitcoin/src/secp256k1/src")
             .flag_if_supported("-Wno-unused-function") // some ecmult stuff is defined but not used upstream
             .define("SECP256K1_BUILD", "1")
             // Bitcoin core defines libsecp to *not* use libgmp.
@@ -38,34 +44,39 @@ fn main() {
             .define("USE_SCALAR_INV_BUILTIN", "1")
             // Technically libconsensus doesn't require the recovery feautre, but `pubkey.cpp` does.
             .define("ENABLE_MODULE_RECOVERY", "1")
+            .define("ECMULT_WINDOW_SIZE", "15")
+            .define("ECMULT_GEN_PREC_BITS", "4")
+            .define("ENABLE_MODULE_SCHNORRSIG", "1")
+            .define("ENABLE_MODULE_EXTRAKEYS", "1")
             // The actual libsecp256k1 C code.
             .file("depend/bitcoin/src/secp256k1/src/secp256k1.c");
 
         if is_big_endian {
-            base_config.define("WORDS_BIGENDIAN", "1");
+            secp_config.define("WORDS_BIGENDIAN", "1");
         }
 
         if use_64bit_compilation {
-            base_config
+            secp_config
                 .define("USE_FIELD_5X52", "1")
                 .define("USE_SCALAR_4X64", "1")
                 .define("HAVE___INT128", "1");
         } else {
-            base_config.define("USE_FIELD_10X26", "1").define("USE_SCALAR_8X32", "1");
+            secp_config.define("USE_FIELD_10X26", "1").define("USE_SCALAR_8X32", "1");
         }
     }
 
-    let tool = base_config.get_compiler();
+    secp_config.compile("libsecp256k1.a");
+
+    let tool = consensus_config.get_compiler();
     if tool.is_like_msvc() {
-        base_config.flag("/std:c++14").flag("/wd4100");
+        consensus_config.flag("/std:c++17").flag("/wd4100");
     } else if tool.is_like_clang() || tool.is_like_gnu() {
-        base_config.flag("-std=c++11").flag("-Wno-unused-parameter");
+        consensus_config.flag("-std=c++17").flag("-Wno-unused-parameter");
     }
 
-    if target.contains("windows") {
-        base_config.define("WIN32", "1");
-    }
-    base_config
+    consensus_config.cpp(true)
+        .include("depend/bitcoin/src")
+        .include("depend/bitcoin/src/secp256k1/include")
         .file("depend/bitcoin/src/util/strencodings.cpp")
         .file("depend/bitcoin/src/uint256.cpp")
         .file("depend/bitcoin/src/pubkey.cpp")