Move add_builtin to SVM

[LucasSte]

Problem

Presently, external SVM users must register builtins in two different places separately: the program cache and a vector passed to load_and_process_sanitized_transactions. These steps are burdensome and can lead to potential mistakes. In addition, the execution of programs should be self contained in the SVM, so the registration of builtins is semantically related to the SVM scope.

Summary of Changes

1. Moved add_builtin from bank.rs to the SVM folder.
2. add_builtin_account is now part of the trait TransactionProcessingCallback.
3. Moved builtin_program_ids from struct Bank to struct TransactionBatchProcessor.

Fixes #303

[LucasSte]

@pgarg66 mentioned he wanted to have a look at these changes, so I'll wait for his review before merging.

[Lichtso]

There is still a todo!() left over here.

[pgarg66]

@LucasSte if this is an optional trait method, maybe have a default implementation of it in the trait definition?

[LucasSte]

It is an optional method, but coming up with a default implementation is not straightforward. In order to add a builtin account, we need access to a generic accounts database, which developers will only provide when they create their own struct to replace Agave's Bank.

[pgarg66]

The default implementation can be empty. So you don't need to implement for test with a todo in it. If the users of SVM do not need builtin they can ignore it.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 81.9%. Comparing base (c0be86d) to head (2406159).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #547     +/-   ##
=========================================
- Coverage    81.9%    81.9%   -0.1%     
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  Files         851      851             
  Lines      230475   230485     +10     
=========================================
- Hits       188785   188774     -11     
- Misses      41690    41711     +21