Subcontext fixes and formatting
Moved is_parser trait to its own file
Added type-erased parser, tests and examples
Merge branch 'develop'

Conflicts:
	include/boost/spirit/home/x3/core/parser.hpp
Merge branch 'origin/x3-devel'
Merge branch 'develop' into x3-devel
Merge branch 'develop' into x3-devel
Fix for #8537: Qi `skip` fails when nested under two or more levels of `lexeme`
Merge pull request #15 from jamboree/develop

fix transform_params/ fix integer_sequence for clang (reverted from commit 29fea725763e27067f5bdda172d790bd0bda5623)
new facade-based API (reverted from commit 7f7dbe90368a18d977e2c8aea313b0f1e57e016b)
Merge branch 'develop'
removing non-variadics code
ading is_rule trait
better x3::variant interface
added ast utilities
removed extra include
added with directive
porting calc7 to x3
implemented raw directive
restarting based on calc6