Support constants in patterns (fixes #696)

[udoprog]

This adds support for using const values as patterns.

The supported value types are:

• booleans, like true.
• characters, like 'a'.
• bytes, like b1.
• strings, like "Hello World"
• byte strings, like #"Hello World".
• integers, like 42
• tuples, like (1, 2, 3, true).
• vectors, like [1, 2, 3, ].
• anonymous objects, like #{foo: 12}

... or any arbitrarily complex combination of the above.

We can't support constant structs and enums (yes) due to the lack of representation in ConstValue, but this might be solvable in the future.