Eliminate Vmeta

Summary: It was useful during transition to vasm, when we needed to
track program points across multiple Vunits. No longer useful.

Reviewed By: @swtaarrs

Differential Revision: D1631688
Don't pass userdata to callback in array_walk unless it was specified

Summary: Fixes #3022

Reviewed By: @ptarjan

Differential Revision: D1634275
Move CodeGenerator::m_unit and m_frozen to CodegenState

Reviewed By: @jdelong

Differential Revision: D1631629