Improve performance and reduce allocations in TemplateBinder ctor

[drieseng]

Improves performance and reduce allocations in TemplateBinder ctors:

• Avoids looping multiple times over parameter policies, and no longer uses Linq.
• Only filters out parameter names from defaults if there are defaults.
• When assigning slots, obtains parameter count only once.

Result:

Method	Version	Mean	Error	StdDev	Op/s	Gen 0	Allocated
ParametersAndDefaultsAndRequiredKeysAndPolicies	master	768.0 ns	5.927 ns	5.544 ns	1,302,060.6	0.0105	968 B
ParametersAndDefaultsAndRequiredKeysAndPolicies	PR - List\	589.1 ns	5.514 ns	5.158 ns	1,697,611.7	0.0095	840 B
ParametersAndDefaultsAndRequiredKeysAndPolicies	PR - ArrayBuilder\	565.9 ns	1.284 ns	1.072 ns	1,767,084.3	0.0076	760 B
ParametersAndNoDefaultsAndRequiredKeysAndPolicies	master	637.8 ns	6.036 ns	5.351 ns	1,567,837.6	0.0095	824 B
ParametersAndNoDefaultsAndRequiredKeysAndPolicies	PR - List\	371.5 ns	3.651 ns	3.415 ns	2,691,922.5	0.0076	696 B
ParametersAndNoDefaultsAndRequiredKeysAndPolicies	PR - ArrayBuilder\	349.1 ns	2.952 ns	2.616 ns	2,864,878.1	0.0072	616 B
NoParametersAndDefaultsAndRequiredKeysAndPolicies	master	690.6 ns	3.484 ns	3.089 ns	1,448,016.1	0.0105	984 B
NoParametersAndDefaultsAndRequiredKeysAndPolicies	PR - List\	483.5 ns	9.588 ns	8.968 ns	2,068,154.3	0.0095	856 B
NoParametersAndDefaultsAndRequiredKeysAndPolicies	PR - ArrayBuilder\	458.9 ns	3.747 ns	3.505 ns	2,179,081.6	0.0086	776 B
NoParametersAndNoDefaultsAndRequiredKeysAndNoPolicies	master	386.1 ns	2.847 ns	2.523 ns	2,590,204.2	0.0048	424 B
NoParametersAndNoDefaultsAndRequiredKeysAndNoPolicies	PR - List\	316.5 ns	3.476 ns	3.252 ns	3,159,387.1	0.0052	504 B
NoParametersAndNoDefaultsAndRequiredKeysAndNoPolicies	PR - ArrayBuilder\	300.6 ns	2.025 ns	1.795 ns	3,326,396.0	0.0043	424 B

Benchmark is available here.

[drieseng]

@JamesNK Do you only ever intend to use the first parameter transformer? In DefaultLinkGenerator.CreateTemplateBinder(RouteEndpoint endpoint) we only pass the first one to the TemplateBinder ctor.

Also, why not pass the constraints and transformer(s) separately to the TemplateBinder ctor? Right now, we do the work of extracting them from the list of parameter policies twice.

[JamesNK]

Do you only ever intend to use the first parameter transformer?

Yes, for simplicity.

Also, why not pass the constraints and transformer(s) separately to the TemplateBinder ctor? Right now, we do the work of extracting them from the list of parameter policies twice.

It is a public constructor. If additional usages of policies in the future then the constructor would need to change.

[natemcmaster]

@rynowak @JamesNK We're merging Routing today into aspnet/AspNetCore. If you can merge this in the next 30 minutes or so, we can bring this along. Otherwise, you'll have to open a new PR against https://github.com/aspnet/AspNetCore.