Declare "digits" static

This removes "digits" from the symbol table of the resulting object file,
which seems like a likely candidate for causing link errors with GHC.
Fix export of module Data.ByteString.Builder.ASCII

Without this fix,  things such as word32Hex appear in the Haddocks but
aren't actually exported from Data.ByteString.Builder,  or indeed
available outside the package at all.
Implement (lazy) byteStringHexFixed

These identifiers are exported from the bytestring-builder shipped
with GHC 7.6, but these functions were renamed so a simple module
export does not suffice.