[#738] Fix `fieldAnnQ` and related quasiquoters

Description

fieldAnnQ and related quasiquoters for Michelson annotations fail to compile on empty strings. The compilation failure is about [] not being a string literal - this prevents overloading the value to Text (which is what UnsafeAnnotation expects).

This proposes the solution, that is suggested in the issue description - adds fromString for the expression quotations.

Related issue(s)

Resolves #738 (closed)

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