Use character literals instead of integer hex values (i.e. `'\x5b','\x0a', ...` instead of `0x5b, 0x0a, ...`) for generated headers.
This avoids C++11 narrowing warnings in a more concise way than using explicit char casts.
Extra whitespace is also removed between elements for brevity.
Tested the performance with:
> time cmake -DJSON_SOURCE_PATH=src/secp256k1/src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.json -DHEADER_PATH=build/after/ecdsa_secp256k1_sha256_bitcoin_test.json -P cmake/script/GenerateHeaderFromJson.cmake
Before:
> 3.57s user 6.01s system 94% cpu 10.136 total
After:
> 0.17s user 0.01s system 98% cpu 0.187 total
