Avoid unnecessary memory allocations in decodeMapFromStruct

[jsamunderu]

Splits allocates memory. This burdens the garbage collector.
By using string.Index to parse string you will be more cache
aware and do not create gabbage.

@mitchellh Can you please review and merge if good enough.

[mitchellh]

Thanks this seems good to me. It always bothered me that we have to range over those.

Realistically from a performance perspective my guess is that this doesn't matter, since reflect in general is very slow. So unless we're doing something really dumb, no optimizations will overcome that most likely.