variable signing precompute table

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

README.md

@@ -61,3 +61,12 @@ libsecp256k1 is built using autotools:
     $ make
     $ make check
     $ sudo make install  # optional
+
+Exhaustive tests
+-----------
+
+    $ ./exhaustive_tests
+
+With valgrind, you might need to increase the max stack size:
+
+    $ valgrind --max-stackframe=2500000 ./exhaustive_tests