Squashed 'src/secp256k1/' changes from b0210a9..bccaf86

bccaf86 Merge pull request #150
2a53a47 Merge pull request #151
5f5a31f Merge pull request #149
3907277 Merge pull request #142
a3e0611 Enable tests in x86 travis builds
45da235 x86 builder
8bb0e93 Merge pull request #155
971fe81 build: fix openssl detection for cross builds
f22d73e Explicitly access %0..%2 as 64-bit so we use the right registers for x32 ABI
e66d4d6 Avoid the stack in assembly and use explicit registers
cf7b2b4 Fix ECDSA message hashes to 32 bytes
056ad31 Really compile with -O3 by default
74ad63a Merge pull request #146
9000458 Merge pull request #145
1f46b00 build: fix __builtin_expect detection for clang
aaba2e0 Merge pull request #136
8a0775c Merge pull request #144
ee1eaa7 Merge pull request #141
c88e2b8 Compile with -O3 by default
6558a26 Make the benchmarks print out stats
000bdf6 Rename bench_verify to bench_recovery
7c6fed2 Add a few more additional tests.
992e03b travis: add clang to the test matrix
b43b79a Merge pull request #143
e06a924 Include time.h header for time().
8d11164 Add some additional tests.
3545627 Merge pull request #118
6a9901e Merge pull request #137
376b28b Merge pull request #128
1728806 Merge pull request #138
a5759c5 Check return value of malloc
39bd94d Variable time normalize
ad86bdf Merge pull request #140
54b768c Another redundant secp256k1_fe_normalize
69dcaab Merge pull request #139
1c29f2e Remove redundant secp256k1_fe_normalize from secp256k1_gej_add_ge_var.
2b9388b Remove unused secp256k1_fe_inv_all
f461b76 Allocate precomputation arrays on the heap
b2c9681 Make {mul,sqr}_inner use the same argument order as {mul,sqr}
6793505 Convert YASM code into inline assembly
f048615 Rewrite field assembly to match the C version
3ce74b1 Tweak precomputed table size for G

git-subtree-dir: src/secp256k1
git-subtree-split: bccaf86caa9c44166e5a66600b742c516e03c3f0
Update libsecp256k1
Update Bitcoin for libsecp256k1 API change
signrawtransaction: validate private key.
depends: teach 'make download' to download sources for all hosts
depends: Move source stamps to source dir and misc cleanups

Since the last commit will force rebuilds of all depends, take the opportunity
to clean up a few other things that would trigger rebuilds as well.

- Move source stamps to the sources dir so that SOURCES_PATH is respected for
  "make download".
- Only print "fetching..." when actually downloading a file.
- Avoid using non-deterministic paths for the recipe hash (patch location).
  This should ensure that all builders get the same resulting build-ids.
- Use a per-package source paths. This will allow for removing old source files
  in the future.
- Use a host-agnostic path for downloads which gets cleaned up properly.
depends: fix packages with hard-coded SOURCES_PATH

Also fixes a nasty bug that removes the downloaded comparisontool jar file.
docs: release process fixups

Add instructions for manually fetching sources, as well as some misc. fixes.
Merge pull request #5457

830ee48 Update Bitcoin for libsecp256k1 API change (Pieter Wuille)
ecae2ac Squashed 'src/secp256k1/' changes from b0210a9..bccaf86 (Pieter Wuille)
Merge pull request #5465

e27d7cb docs: release process fixups (Cory Fields)
d7db4b6 depends: fix packages with hard-coded SOURCES_PATH (Cory Fields)
d546191 depends: Move source stamps to source dir and misc cleanups (Cory Fields)
422f873 depends: teach 'make download' to download sources for all hosts (Cory Fields)
Merge pull request #5461

aa768f1 signrawtransaction: validate private key. (Pavel Janík)
[Qt] ensure cert errors lead to rejecting payment requests

- extend PaymentRequestPlus::getMerchant to return status of payment
  request errors related to certificates/PKIs etc.
- do a real check for errors, which was missing until now, in
  PaymentServer::processPaymentRequest and also extend our tests in
  paymentservertests.cpp to not only rely on the content of "merchant"
  passed to getMerchant
- this explicitly treats insecure payment requests not as errors, as they
  are allowed as per BIP70