Move as much data as possible into non-writable text by declaring it const

While reviewing this for threadsafety I noticed a lot of internal tables were in the (read/write) data section instead of the (read-only) text section. This pull request declares everything that is actually constant as 'const', which both makes it clear to the reader that there is not a threadsafety issue, and helps the compiler enforce this invariant, and is a (slight) performance win. This pull request touches a lot of lines but each individual commit should be small enough to review easily.

Most of the remaining global mutable variables are the `asn_TYPE_descriptor_t` and `asn_TYPE_member_t` structures, since those actually are fixed up at runtime for inherited types.