Seal types not suitable for extending

[lahma]

CLR can make better decisions when it knows there's no worry to do virtual calls to ensure that possible overridden method would be correctly called. Types really don't have extension points via inheritance.

Fluid.Benchmarks.FluidBenchmarks

Diff	Method	Mean	Error	Allocated
Old	Parse	3.193 μs	0.0312 μs	2.68 KB
New		3.152 μs (-1%)	0.0307 μs	2.68 KB (0%)
Old	ParseBig	16.892 μs	0.2980 μs	11.61 KB
New		16.840 μs (0%)	0.2747 μs	11.61 KB (0%)
Old	Render	133.681 μs	2.5941 μs	95.86 KB
New		130.399 μs (-2%)	1.4315 μs	95.86 KB (0%)
Old	ParseAndRender	135.980 μs	1.5126 μs	99.01 KB
New		135.876 μs (0%)	2.0597 μs	99.01 KB (0%)

[sebastienros]

I fear that it will break someone. But let's try and see people complain, the current version being quite stable they shouldn't need to upgrade and won't be blocked.

[lahma]

This was my thinking too, let's break it and unseal when needed.