Use input and local keywords instead of let in declarations

[eemeli]

Implements the changes proposed in #469. Submitting this as a draft until that PR is merged as accepted.

The expression rule is here redefined as

expression = literal-expression / variable-expression / function-expression
literal-expression = "{" [s] literal [s annotation] [s] "}"
variable-expression = "{" [s] variable [s annotation] [s] "}"
function-expression = "{" [s] annotation [s] "}"

in order to make it easier for human readers, and to make variable-expression reusable in input-declaration. This isn't strictly speaking LL1, but it may be transformed to its earlier shape to make it LL1 again.

[eemeli]

Merged with the modification agreed to on today's call.