Lazy-load HTML templates

[andrew]

Move template parsing from server startup into a sync.Once guarded load() method called on first Render(). API and proxy endpoints never touch templates, so they no longer pay the ~780µs startup cost.

NewTemplates() is replaced by a zero-value &Templates{}. Parsing happens once on the first HTML page request.

Benchmarks (before → after):

                 │   before    │            after             │
                 │   sec/op    │   sec/op    vs base          │
ServerCreate-8     736.1µ ± ∞    2.073n ± ∞  -100.00%
TemplatesParse-8   734.4µ ± ∞    783.3µ ± ∞       ~
FirstRender-8      947.3µ ± ∞    988.6µ ± ∞       ~

Closes #53