Drop the `terse` keyword in favor of a `@concrete_terse` macro

[jonniedie]

The terse keyword is weird and nonstandard and, more importantly, is messing with other macros like Base.@kwdef.

[jonniedie]

Turns out it's not terse that's making it not work with Base.@kwdef it's the fact that both macros are defining things that aren't struct definitions (in the case of @concrete terse it's the show methods, with Base.@kwdef it's the outer constructor`). We'll have to handle this on our side.