Remove calls to impure mathlib functions.
Add README

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Make std.string.indexOf @safe
Mark std.string.indexOf as @safe
do reverse of a bytecode in lookbehinds
backtracking matcher works with reversed bytecode
ditto with Thompson matcher
cleanup to keep lookahead/lookbehind more in sync for future

Fix 64 bit issues, remove useless debug hook
Replace trusted lambda block with trusted nested function
Implement pure floor()/ceil().
Implement pure tan().
Implement pure atan().
Implement pure atan2().
Implement pure exp().
Implement pure expm1().
Implement pure exp2().
Implement pure log().
Implement pure log10().
Implement pure log1p().