PE: TLS parser rework

[kkent030315]

• PE: TLS parser generics and bitness-related rework #433

Changelists:

• Removed generics from ImageTlsDirectory::parse* and TlsData::parse*, instead added new argument is_64 as proposed in the issue.
• Added slice bound check in TlsData::raw_data parser to compliant with panic-free design in goblin.

I personally prefer not making ImageTlsDirectory32―as it's more comfortable than the way duplicating the same kind of structs and merging to 64-bit as proposed in the issue.

This change should be marked as breaking-change and I expect this to be rolled up in 0.10.