[patch] forward declare core types (pmc, string, interpreter) to avoid using different declarations in core and extensions

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This patch adds a header file to forward declare string, pmc and interpreter types to avoid the use of different declarations in core and in extensions/embedding, allowing better type checking by C and C++ compilers. Also moves Parrot_compile_string to embed.h as suggested in the embed draft pdd.

This change will allow embedding without including inappropriate headers or fake PARROT_IN_CORE defines.

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The plan is to commit this patch after the release. If someone has any problem with this, please report as soon as possible.

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Applied in r36847

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Several days passed without problems reported, closing ticket.