Configurable var transformers & better ellipsis grouping for Turnstile#11

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iitalics wants to merge 4 commits intostchang:masterfrom iitalics:turnstile-new-clauses

+570-193

Commits on Jun 8, 2017

var-transformer configurable thru #:var-stx keyword to (infer ...)
iitalics
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rewrote turnstile clause syntax; more intuitive + access to var-stx
iitalics
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Commits on Jun 12, 2017

added phase-0 syntax define-typed-variable-syntax
iitalics
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Commits on Jun 13, 2017

create example linear language + tests
iitalics
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