allow any expression in a matrix literal (fixes #4)
MatrixLiteral: infer type from entries if unspecified
fix interpretation to allow any expression in matrix literal
fix imaginary literals

Allowing any expression in matrix literals somehow
caused problems with recognizing imaginary literals,
mistaking them for numbers and the variable i. Fix
this by matching ImaginaryLiteral before CoefficientLiteral.
Merge branch 'any-expr-literal'