Avoid dyn in to_html

[kornelski]

Makes benchmarks slightly better:

 name                before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 escaped_long        75,357          70,066               -5,291   -7.02%   x 1.08
 escaped_no_op       92,623          74,609              -18,014  -19.45%   x 1.24
 escaped_nums        104,926         81,901              -23,025  -21.94%   x 1.28
 escaped_short_dyn   123,600         105,811             -17,789  -14.39%   x 1.17
 escaped_short_impl  113,133         90,689              -22,444  -19.84%   x 1.25
 raw                 27,895          26,434               -1,461   -5.24%   x 1.06

(the comparison is with improved benchmark code in both cases)

[kornelski]

Blocked by #57

[kaj]

Hi! Could you rebase now that #57 is fixed and #55 is merged? I'm sorry for the long delay in the later.

[kaj]

Hm. One probem with this is that it makes it impossible to handle a dyn ToHtml, which I use in some examples and think is a good thing in some real cases. Is the performance improvement worth that?

On the other hand, this would improve consistency between a template and a ToHtm impl, making them take the same kind of output argument, which would probably make #88 easier to fix.

[kornelski]

You're right. Object safety is a feature.