Allow overriding CRYSTAL_PATH in the wrapper script

[bcardiff]

This will allow setting the CRYSTAL_PATH in the same fashion as in official packages.

CRYSTAL_PATH=/path/to/prepend:$(bin/crystal env CRYSTAL_PATH)

It is mainly convenient for test-ecosystem to reduce the noise on specific deprecation warnings by overriding some files of the std-lib.

[jhass]

Mmh, feels like a potential footgun. bin/crystal is intended to run the compiler in the local repo and thus heavily relies on $CRYSTAL_ROOT/src being inside the path. I can easily imagine setups, say version switchers akin to RVM or dependency managers competing with shards, that make use of setting CRYSTAL_PATH globally and thus break working on the compiler repo in subtle ways.

Maybe this should either ensure $CRYSTAL_ROOT/src to be in CRYSTAL_PATH (but where to append it? Hard problem.) or just error out or at least warn if its not.

[bcardiff]

I like the proposal of warning if $CRYSTAL_ROOT/src is not mentiones in CRYSTAL_PATH

[bcardiff]

Something I forgot to mention is that this allows shards specs to work with the wrapper script since in their spec the CRYSTAL_PATH is set.