Change tree type in Router struct

[bmf-san]

Description

The tree that Router has might as well be a map.

tree                 map[string]*tree   // key is http method

cf. https://github.com/bmf-san/goblin/blob/master/router.go#L11

Describe the feature you'd like to request

For data structure optimization.

Describe the solution you'd like

Need to prove efficency by benchmark tests.