Improve pmc2c to support :manual_wb flag.

[bacek]

Hello.

Currently pmc2c in gen_gc branch generates automatic write barriers for:

1. VTABLEs marked with :write pragma. 2. All METHODS.

It's quite suboptimal. For example FIA.set_foo_keyed_int shouldn't trigger WB. And many of METHODs aren't mutating state of PMC.

To improve it we have to suppress generation of write barriers when pragma :manual_wb is specified.

-- Bacek

[bacek]

Hello.

gen_gc branch now have support for :manual_wb on VTABLEs. FIA is sample how it's can be used.

-- Bacek

[cotto]

Awesome. Once it's documented and the branch is merged into master, we can close this ticket.

[bacek]

Not so fast. Support for METHODs isn't there yet :)

[Vasily Chekalkin]

Implement :manual_wb for METHODs. Closes #2018.

Changeset: db775476eaeed3a3be00ba5f4efd7d15e06c1155