`aws-smithy-http-server`: implement basic URI pattern conflict disambiguation (#1036)

This commit implements basic URI pattern conflict disambiguation when
routing incoming requests to service operations.

It does this by introducing a measure of how "important" a `RequestSpec`
is. The more specific a `RequestSpec` is, the higher it ranks in
importance. Specificity is measured by the number of segments plus the
number of query string literals in its URI pattern, so
`/{Bucket}/{Key}?query` is more specific than `/{Bucket}/{Key}`, which
is more specific than `/{Bucket}`, which is more specific than `/`.

Why do we need this?

Note that:

1. the Smithy spec does not define how servers should route incoming
   requests in the case of pattern conflicts; and
2. the Smithy spec even outright rejects conflicting patterns that can
   be easily disambiguated e.g. `/{a}` and `/{label}/b` cannot coexist.

We can't to anything about (2) since the Smithy CLI will refuse to build
a model with those kind of conflicts. However, the Smithy CLI does allow
_other_ conflicting patterns to coexist, e.g. `/` and `/{label}`. We
therefore have to take a stance on (1), since if we route arbitrarily we
render basic usage impossible (see issue #1009).

So this ranking of routes implements some basic pattern conflict
disambiguation with some common sense. It's also the same behavior that
the TypeScript sSDK is implementing [0].

This commit also:

* makes the `future` module private,
* hides documentation for the `request_spec` module; and
* makes some fields from some structs in the `request_spec` module
  private and adds constructors.

Closes #1009.

[1]: https://github.com/awslabs/smithy-typescript/blob/d263078b81485a6a2013d243639c0c680343ff47/smithy-typescript-ssdk-libs/server-common/src/httpbinding/mux.ts#L59.