Data representation of generated_bignum_stubs

Under the measurement conditions of #793 the third largest function in a 32-bit-targeting compiler is to_word32Prog_word_bignum_generated_bignum_stubs (595812 bytes). Since this is executed once per compile, it's a reasonable candidate for encoding as data and unpacking using a function along similar lines to #774.

Start by reading issues #793 and #774, then inspect the generated_bignum_stubs path containing to_word32Prog_word_bignum_generated_bignum_stubs. Determine how the existing function is represented and how #774 unpacks analogous data; done means the representation is encoded as data, unpacked during compilation, and preserves the current behavior and measured benefit.

Written by the indexing model from the issue text.