Filter selector – const atom comparisons [?<path> <binop> <atom>]

[V0ldek]

Is your feature request related to a problem? Please describe.
After #154, we can extend filters to test against a constant atomic value.

Describe the solution you'd like
This is easy, as the spec requires codepoint, bitwise comparison of strings, so no unicode handling comes into play. We do need parsing of integer values inside the JSON, since floating point comparison is not easy to do without actually parsing the number.

Additionally, there are "dumb" filters that should be handled this as well, like [?42 == 42] or [?42 != 42], so either always satisfied or never satisfied. These can just be collapsed to either not emitting a filter at all, or emitting an empty query matching nothing.

Additional context
RFC draft.

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