Add API function to retrieve size of bijector output from bijector input

[sethaxen]

As originally noted in #263 (comment), the following lines assume that inputs and outputs of a bijector have the same shape, which is no longer required:

• Bijectors.jl/src/bijectors/stacked.jl

  Line 77 in fc155c9

  @assert size(y) == size(x) "x is size $(size(x)) but y is $(size(y))"

• Bijectors.jl/src/transformed_distribution.jl

  Line 95 in fc155c9

  Base.size(td::Transformed) = size(td.dist)

To fix this, these methods need an API function that computes the output size of a bijector from the input size, defaulting to the same size.

Alternatively, if Bijectors is meant to support non-array types, such as arbitrary Structs, then these size checks make no sense, and it is perhaps better to generalize this code.

[sethaxen]

Fixed by #270