variable signing precompute table

Summary:
make ECMULT_GEN_PREC_BITS configurable

ecmult_static_context.h: add compile time config assertion (#3) - Prevents accidentally using a file which was generated with a
different configuration.

README: mention valgrind issue

With --with-ecmult-gen-precision=8, valgrind needs a max stack size
adjustment to not run into a stack switching heuristic:

http://valgrind.org/docs/manual/manual-core.html

-max-stackframe= [default: 2000000]
The maximum size of a stack frame. If the stack pointer moves by more than this amount then Valgrind will assume that the program is switching to a different stack.

You may need to use this option if your program has large stack-allocated arrays.

basic-config: undef ECMULT_WINDOW_SIZE before (re-)defining it

This is a backport of libsecp256k1 PR337

Test Plan:
Run make and cmake build and check the pass.
Pass random values to SECP256K1_ECMULT_GEN_PRECISION and check that cmake fails.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5362