string typedef: Fix fmt import and return type bugs

[r-hang]

PR (#554) introduced an optimization for the String() method of typedefs.

There are two issues with this change:

• Since the "fmt" import was global, if fmt.Sprint was dropped from the typedef code generation the import would be unused.
• Using compiled.RootTypeSpec for isStringType results in situations where a non-string type is return from a String() function resulting in uncompilable code. func(v StringReDef) String() string { x := (stringdef.StringDef)(v) return x } This happens when a type is a typedef of another string typedef.

I've added generated code cases to demonstrate and verify the corrected behavior.

[codecov[bot]]

Codecov Report

Merging #568 (a9f46ab) into dev (f4b1af6) will decrease coverage by 0.10%. The diff coverage is 12.82%.

@@            Coverage Diff             @@
##              dev     #568      +/-   ##
==========================================
- Coverage   68.09%   68.00%   -0.10%     
==========================================
  Files         139      140       +1     
  Lines       23838    23872      +34     
==========================================
  Hits        16233    16233              
- Misses       4544     4578      +34     
  Partials     3061     3061              

Impacted Files	Coverage Δ
gen/internal/tests/collision/collision.go	65.63% <ø> (ø)
gen/internal/tests/nozap/nozap.go	52.85% <ø> (ø)
gen/internal/tests/set_to_slice/set_to_slice.go	56.99% <ø> (ø)
gen/internal/tests/stringdef/stringdef.go	0.00% <0.00%> (ø)
gen/internal/tests/structs/structs.go	63.74% <ø> (ø)
gen/internal/tests/typedefs/typedefs.go	67.07% <4.76%> (-1.05%)	:arrow_down:
gen/internal/tests/services/services.go	61.30% <100.00%> (ø)
gen/internal/tests/uuid_conflict/uuid_conflict.go	63.44% <100.00%> (ø)
gen/typedef.go	100.00% <100.00%> (ø)

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