Rearrange exports; deprecate lambdamin, lambdamax

[ericphanson]

I'm thinking about the API we should expose in Convex.jl v0.13 (see #346), and realized it's hard to know what API we expose now, since the exports are scattered across files.

I've therefore moved them all to the top of src/Convex.jl. I also unexported many functions that I feel should be internal:

• All types corresponding to Atoms (these should be constructed via functions, e.g. using + instead of AdditionAtom)
• All types corresponding to constraints and vexities (these likewise be automatically constructed from the exposed API)
• Base.to_index, promote_size, get_row, conic_form!, Value, ValueOrNothing, which just don't seem to need to be exported
• Functions already exported from Base

I have left the re-exports from LinearAlgebra, though maybe we should revisit that (maybe a user should need to do using LinearAlgebra in order to get e.g. norm and eigmin).

I also deprecated lambdamin and lambdamax in favor of LinearAlgebra.eigmin and LinearAlgebra.eigmax, which closes #336.