non string attributes fails silly compilation

mir-algorithm uses user defined attributes that may not be a string.

For example:

../../../.dub/packages/silly-0.7.0/silly/silly.d(271,11): Error: cannot implicitly convert expression llvmFastMathFlag("fast") of type llvmFastMathFlag to string
../../../.dub/packages/silly-0.7.0/silly/silly.d(77,44): Error: template instance `silly.getTestName!(__unittest_L518_C6)` error instantiating

Would be nice to use silly with Mir.

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