Use let expressions in encoding to reduce redundancy

[Felalolf]

This should be merged after: #531

This PR is a first attempt to make use of let expressions in the encoding more often.
The main contribution of this PR are the value function of the type encoding and the two bind functions.

Currently, it is unclear at which points we should transform an expression writer to an expression, and thus "discharge" the accumulated let expressions. For now, my idea was to use pure whenever one of the new bind methods is used in the encoding of expressions. An alternative is, similar to statements, to use pure at finalExpression in the type encoding.