Feature completion for load/store. Merge atomic load/store into regular load/store.

[ARF1]

Completes implementations of load and store to support all features. Merges atomic load/store with regular load/store.

Fixes #450.

Incorporated PRs:

• closes Merge atomic load/store into regular load/store #559
• closes Set AppVeyor to python 3.6 #565

This builds on @ssikdar1's works in #559 where he merged the public api of atomic load/store into regular load/store.

Further commits make the following changes to @ssikdar1's work:

• restore the deleted public api for atomic memory access and add deprecation warnings to builder.load_atomic() and builder.store_atomic()
• merge LoadAtomicInstr with LoadInstr and StoreAtomicInstr with StoreAtomicInstr
• support volatile flag for load/store
• support sync_scope for atomic operations
• document api additions & deprecation warnings for obsolete api

[ARF1]

@stuartarchibald This is ready for review. Thanks.