Added ConvertTo<*> methods in exec.cpp, and eval_<*> methods in express.cpp
Added Eval(Optional)ExprAs<Type>() methods in exec.cpp, where <Type> = BooleanRef, DataRef, etc
Updated MCKill::exec(ep) to MCKill::exec_ctxt(ctxt)
Updated MCRead::exec(ep) to MCRead::exec_ctxt(ctxt)
Merge remote-tracking branch 'upstream/refactor-syntax_unicode_shims' into refactor-syntax_unicode_shims3

Conflicts:
	engine/src/exec.cpp
	engine/src/exec.h
Merge remote-tracking branch 'upstream/refactor-syntax_unicode_shims' into refactor-syntax_unicode_shims3
Replaced calls to EvalOptionalExprAsStringRef() with EvalOptionalExprAsNullableStringRef()
Merge remote-tracking branch 'upstream/refactor-syntax_unicode_shims' into refactor-syntax_unicode_shims3
Replaced EvalOptionalExprAsNameRef() calls with EvalOptionalExprAsNullableNameRef()
Updated MCSeek::exec(ep) to MCSeek::exec_ctxt(ctxt)
Updated MCWrite::exec(ep) to MCWrire::exec_ctxt(ctxt)
Merge branch 'refactor-syntax_unicode_shims3' into refactor-syntax_unicode_shims
[[ RefactorSyntax ]] Removed extra addition of MCExecContext::SetIt on merging
MCresult calls replaced with ctxt . SetTheResultTo
MCeerror->add calls replaced with ctxt . LegacyThrow()