Runtime: Expose builtin program IDs to crate

[buffalojoec]

This is chunk 2/7 of the broken-up PR #79.

Problem

As we prepare to migrate builtin programs to Core BPF, it's important to decouple
components such as snapshot minimization from the static BUILTINS list.

By adding a crate-only method to Bank that will allow a caller to immutably
borrow the bank's list of builtin program IDs, other runtime components can
get the bank's current list of supported builtins.

Summary of Changes

Two commits:

1. Rename the bank's builtin_programs field to builtin_program_ids since the term "builtin" is heavily overloaded as it is, and this field is only the IDs, not the BuiltinPrototype elements, as seen elsewhere in bank.rs.
2. Expose a crate-only method get_builtin_program_ids(&self) -> &HashSet<Pubkey> on the bank and use it in the snapshot minimizer.

[buffalojoec]

A case can be made for making this method completely public, as suggested in #316.

[CriesofCarrots]

One comment about the metric name. Otherwise this lgtm.
However, it would be good to confirm with @Lichtso that we aren't missing some rationale for removing a similar bank method in solana-labs#31654 (aside from the convenience of using the static list directly).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 81.9%. Comparing base (54575fe) to head (34308e2).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #318     +/-   ##
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- Coverage    81.9%    81.9%   -0.1%     
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  Files         838      838             
  Lines      226927   226934      +7     
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- Hits       185901   185899      -2     
- Misses      41026    41035      +9     

[CriesofCarrots]

Wfm, but please check in with @Lichtso /the runtime team before merge.