Merge #830: Rip out non-endomorphism code + dependencies

c582abade1c50ef50dc7ee9f7b7af8e06e22065d Consistency improvements to the comments (Pieter Wuille)
63c6b71616816b19bec9cb3ab6b45ae5afd955f0 Reorder comments/function around scalar_split_lambda (Pieter Wuille)
2edc514c90293af8f602e4376e832773779c9426 WNAF of lambda_split output has max size 129 (Pieter Wuille)
4232e5b7da0a68adc14fa4b481f7e106403c200d Rip out non-endomorphism code (Pieter Wuille)
ebad8414b0e68041568d0b5ebe0bd395dbfbed9e Check correctness of lambda split without -DVERIFY (Gregory Maxwell)
fe7fc1fda8675aa9d79dae54a1b8b3cd06abcf81 Make lambda constant accessible (Pieter Wuille)
9d2f2b44d895509e8c4e7831fa917f13fa69f054 Add tests to exercise lambda split near bounds (Pieter Wuille)
9aca2f7f07b0563f8c65fcc22a0a91325cf6273b Add secp256k1_split_lambda_verify (Russell O'Connor)
acab934d24ff26289ab9930587c3fc51c30c6a2f Detailed comments for secp256k1_scalar_split_lambda (Russell O'Connor)
76ed922a5f09d63e0622825ca83d9301c1ef3efe Increase precision of g1 and g2 (Russell O'Connor)
6173839c90553385171d560be8a17cbe167e3bef Switch to our own memcmp function (Tim Ruffing)

Pull request description:

  This is a rebased/combined version of the following pull requests/commits with minor changes:
  * #825 Switch to our own memcmp function
    * Modification: `secp256k1_memcmp_var` is marked static inline
    * Modification: also replace `memcmp` with `secp256k1_memcmp_var` in exhaustive tests
    * Modification: add reference to GCC bug 95189
  * #822 Increase precision of g1 and g2
    * Modification: use the new `secp256k1_memcmp_var` function instead of `memcmp` (see https://github.com/bitcoin-core/secp256k1/pull/822#issuecomment-706610361)
    * Modification: drop the " Allow secp256k1_split_lambda_verify to pass even in the presence of GCC bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189." commit, as it's dealt with using `secp256k1_memcmp_var`.
    * Modification: rename secp256k1_gej_mul_lambda -> secp256k1_ge_mul_lambda
  * A new commit that moves the `lambda` constant out of `secp256k1_scalar_split_lambda` and (`_verify`).
  * The test commit suggested here: https://github.com/bitcoin-core/secp256k1/pull/822#issuecomment-706610276
    * Modification: use the new accessible `secp256k1_const_lambda` instead of duplicating it.
  * #826 Rip out non-endomorphism code
  * A new commit that reduces the size of the WNAF output to 129, as we now have proof that the split output is always 128 bits or less.
  * A new commit to more consistently use input:`k`, integer outputs:`k1`,`k2`, modulo n outputs:`r1`,`r2`

ACKs for top commit:
  real-or-random:
    ACK c582abade1c50ef50dc7ee9f7b7af8e06e22065d code inspection, some tests, verified the new g1/g2 constants
  jonasnick:
    ACK c582abade1c50ef50dc7ee9f7b7af8e06e22065d didn't verify the proof

Tree-SHA512: 323a3ee3884b7ac4fa85c8e7b785111b5c0638d718bc1c805a38963c87411e81a746c98e9a42a3e2197ab34a874544de5cc51326955d1c4d0ea45afd418e819f