Step back from Token and Block types

[Thibaut Lienart]

it's a bit annoying but it's explicitly warned against:

https://docs.julialang.org/en/v1/manual/performance-tips/#The-dangers-of-abusing-multiple-dispatch-(aka,-more-on-types-with-values-as-parameters)

[Thibaut Lienart]

should probably move back to having it as a field.

Note that we don't really need to use Symbols everywhere. Since we only effectively have few tokens we could use a basic int encoding like

const T_EOS = 0
const T_NONE = -1
const T_DBB_OPEN = 1
...

the number does not matter. But then we can have bits typing like

struct Token <: AbstractSpan
    id::Int
    from::Int
    to::Int
end

which might just be better full stop...