Add plumbing through ShadyingSystem into Context to support Batched Execution

[AlexMWells]

Add Symbol::wide_dataoffset() to track offset in heap to wide version of symbol's data

Add count to Context to track batch size of last execution

Add template ShadingContext::Batched interface to group together batched versions of execute_init, execute_layer, and execute methods. Instantiate ShadingCOntext::Batched<8> and ShadingContext::Batched<16>.

Refactor std::vector<std::pair<ErrorHandler::ErrCode, std::string> m_buffered_errors to use proper ErrorItem class with optional Mask to identify which data lanes the Error message applies to.

Refactored ShadingContext::process_errors into a templated helper to serialize batched or non-batched errors up to the ShadingSystem.

Modify ShadingContext::symbol_data to use the symbol's wide_dataoffset() when last execution was batched.
Add RunLLVMGroupFuncWide to model a a function pointer to a batched verson of a JIT'd shader function that accepts batched shader globals and an integer representing the mask (active data lanes) to execute on.
To hold onto wide versions of llvm functions side by side with scalar, added llvm_compiled_wide_version, llvm_compiled_wide_init, llvm_compiled_wide_layer.

Added ShadingSystemImpl::Batched interface to group together batched methods jit_group, and jit_all_groups
Instantiate ShadingSystemImpl::Batched<8> and ShadingSystemImpl::Batched<16>
Added ShaderGroup::batch_jitted to track if a group had been jitted for batching (separately from scalar jit)
Added ShaderGroup::llvm_groupdata_wide_size to track amount of heap space needed for a wide version of the group data structure.

Description

Tests

Checklist:

• I have read the contribution guidelines.
• I have previously submitted a Contributor License Agreement.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

LGTM

(Note for readers -- Alex and I went over this in detail over a conference call a few days ago, so I had already made some comments and he some modifications before it got here.)